Suspect

1. If the purchase and sale agreement is removed from the manila folder, and the folder is closed, it can never be reopened; OPEN FOLDER is then treated as EXAMINE FOLDER. If you put something else in the folder before closing it, that object is permanently out of play.
2. It's possible to get naked. If you DROP ALL, reference is made to the "western outfit" you're wearing. However, it doesn't seem to be a 'real' object. EXAMINE OUTFIT works as EXAMINE ME, it doesn't show in your inventory, and if you try to REMOVE or DROP it you're told, "The result would be embarrassing.". But you can THROW OUTFIT. After you do this, DROP ALL no longer includes the outfit, but it doesn't show up in the room description, and you can't GET it. EXAMINE OUTFIT works as LOOK in the location where you threw it, and everywhere else it says, "You can't see the western outfit here." REMOVE OUTFIT still gives, "The result would be embarrassing.", but only where you threw it; elsewhere it says, "You can't see any outfit here." DROP OUTFIT produces, "You don't have the western outfit." EXAMINE ME still mentions the outfit, and no one notices you're naked.
3. The verb EXAMINE can get very confused if you're seated. If you SIT on a sofa, toilet, or chair--except for the window seat in the Morning Room or the armchair in the Library--then you can't EXAMINE anything you're carrying; it says, "You can't see the [object] here." Things get even stranger if you sit on the side table in the Library or the end table in the Sitting Room (which for some reason is the default, if you simply type SIT). Then anything you EXAMINE produces two lines of output, and it claims everything has a telephone sitting on it. Most of the time, the two lines are the same. For example, EXAMINE ME gives, "On yourself sits a telephone." twice, EXAMINE PEN says, "On the pen sits a telephone." twice, EXAMINE TELEPHONE gives, "On the telephone sits a telephone." twice, and EXAMINE LINDA gives, "On Linda sits a telephone, accompanied by a Linda's hair and a ballerina costume." twice. Sitting on one of these tables gives you limited clairvoyance; you can EXAMINE most objects in the game, even if not present: EXAMINE FIREPLACE says, "You can't see any fireplace here. On the such thing sits a telephone.", but EXAMINE LARGE DESK gives, "On the large desk sits a telephone, accompanied by a manila folder." twice. You can get information about things throughout the game, including rooms, by examining them from there. EXAMINE TABLE gives, "On the end [or side] table sits a telephone. On the telephone  sits a telephone.", in the Infocom interpreter (with two spaces after "telephone"). In Frotz, the second line reads, "On the ck it's k feher x sits a telephone." In some cases, you can use IT after such an EXAMINE command to interact with objects that aren't present. For example, if EXAMINE LARGE DESK says, "On the large desk sits a telephone, accompanied by a manila folder." twice, then GET IT takes the folder.
4. You can WEAR the western mask, gunbelt, or reporter's overcoat while it's in an open container you're carrying, and it doesn't get removed from the container. Your inventory shows, for example, "The trash basket contains: a reporter's overcoat (being worn)". You can DROP the container, and LOOK shows the same thing. You can go away and come back, or CLOSE and OPEN the container, and the worn object is still being worn.
5. The reporter's overcoat must really be magic. If you WEAR your coat, you can PUT things in it that you can't otherwise, and you can PUT the coat in things that you can't otherwise.
6. There are a number of ways to PUT each of two containers inside the other, making both disappear. All of them involve either the tool chest or reporter's overcoat. You can do it with the chest and the manila folder, in either order. You can do it with the chest and the hamper, putting the hamper in first. You can do it with the chest and the trash basket, waste basket, or fairy mask, putting the chest in first. All of the ways to do it with your coat take advantage of the "magic coat" bug, and sometimes the "WEARing from containers" bug. You can do it in both orders with your coat and the chest, Alicia's wet overcoat, or the folder, but you have to WEAR your coat once or twice. There are also ways to mutually nest three, four, or even five containers, but six seems to be impossible. I haven't achieved it, but there are so many permutations to try. It seems that in most cases, you can't put something in a "great grandchild" of a container--and you're often more restricted than that. Here's a solution for five items: WEAR MY COAT. PUT MY COAT IN WET COAT. WEAR MY COAT. PUT CHEST IN MY COAT. PUT WET COAT IN FAIRY MASK. PUT FAIRY MASK IN TRASH BASKET. PUT TRASH BASKET IN CHEST. Note that this sequence fails if you REMOVE MY COAT at any point before finishing.
   You can also use this bug to crash the game. Here's an example. If you're holding the tool chest, the manila folder, and the pen, then PUT CHEST IN FOLDER followed by PUT ALL IN CHEST crashes. Frotz exits with, "Fatal error: Stack overflow".
7. If you nest containers too deeply (at least 6) then you get gibberish in your inventory, as in Zork I. In fact, it's possible to nest all 8 of the takeable container objects: reporter's overcoat in wet overcoat in fairy mask in waste basket in tool chest in manila folder in trash basket in wicker hamper (again, you need to WEAR MY COAT a couple of times). Taking inventory after that makes for interesting reading!
8. A number of unexpected objects are closable, openable containers, often with buggy results. The armchair in the Library is an example. You can even PUT Veronica's body in it, and CLOSE it. If you do so, the detective still finds it, but unlike other closed containers, the text doesn't mention the detective looking "inside" the chair, and the chair stays closed, concealing the body from view. Another example is the fairy mask. If you CLOSE and OPEN it before looking in it, you get, "You open the fairy mask and see ." The computer in Veronica's office is a closable, openable container, too. If you PUT things in it, and then say GET ALL FROM COMPUTER, you get a list of its contents, each item followed by, "It's attached with a theft prevention bolt." You can GET each item by name, however. When the bookcase at Media Room is closed, LOOK IN BOOKCASE lists its contents, but using just about any verb other than EXAMINE on something in it says, "You can't see any [object] here." You can PUT things in the closed armchair, secretary, ballroom fireplace, and gunbelt (just the bullet). You can even see objects in the closed ballroom fireplace, but you can't interact with them.
9. From the beginning, if you go WEST and then WAIT for Veronica to leave the ballroom, GET HER produces the same message as GET VERONICA would with her present: "Stop that! Michael, help me!" Michael restrains you, a quizzical look in his eye. This works even after Veronica is dead. You can similarly use FEEL HER, HIT HER, or LISTEN TO HER. JOIN HER works up until the moment of her death--but of course, you get stuck in the hall.
10. You can THROW an object AT any person, present or not. You always get, "[So-and-so], puzzled by your unusual methods, ducks as the [object] flies by." This even works on Veronica after she's dead, as long as you are not in the same room as her body, visible or not. Similarly, you can always SMELL anybody.
11. If you take Veronica's body to the Library, drop it, hide behind the chair, and wait, Colonel Marston notices the body as soon as he comes in, but Michael doesn't notice until after they "glance suspiciously around the room", he locks the doors, and hands Marston the paper. Finally he sees the body, runs to her, etc. Then, suddenly, he turns to you: "Did you do this? I'm calling the police!". But you can still surprise them by coming out from behind the chair. Similarly, if you drop the body in the Garage and hide behind the car, Michael turns to you accusingly, then goes about his secret business in the trunk, since he hasn't really seen you. You can still GET UP and surprise Michael after that. In fact, the same things happen if you're holding the body.
12. When you're hiding behind the armchair for Michael and Colonel Marston's meeting in the Library, it makes little sense just which actions do and don't give away your presence. You can't use OPEN on anything visible, even if you're carrying it, it's already open, or can't be opened. You can't GET anything visible, even if you already have it. You can't use any compass direction, or IN. You can't use ASK, TALK, or address any character by name. Any of these things cause you to "emerge from concealment". On the other hand, you can CLOSE and LOCK the door (locking Michael out doesn't work; he comes right in anyway), UNLOCK both doors, DROP or THROW objects (even AT them!), HIT them, FEEL them, TELL them ABOUT things, ACCUSE them, GIVE or SHOW them things--all producing normal responses--and they continue not knowing you're there. If you SIT on the chair or the table, you come out of hiding without the game mentioning it; they don't react. The same rules apply when you're hiding behind the car from Michael.
13. Using ME as the object confuses a number of verbs. HIT ME responds, "The reporter slaps you right back. It hurts, too." SMELL ME gives, "He smells just like the reporter." MOVE ME claims, "You can't move the reporter." Finally, FEEL ME and READ ME both say, "You can't see yourself here."
14. If you try to interact with an object that's not in the room, getting, "You can't see any [object] here.", then AGAIN responds, "I can't see the such thing anymore.", even if you have now moved to where it is!
15. Although SIT ON TOILET produces, "You are now sitting on the toilet.", GET IN TOILET claims, "You are now in the toilet." Either way, you're sitting on it. This works with anything you can sit on (except the window seat), and it lets you sit on something that has an object on it. For example, you can sit on the side table without removing the book.
16. If you PUT something in a toilet, it's gone forever, unless you remove it using IT right afterward. Although you can't interact with the object, SIT ON TOILET produces, "You might crush the [object].", even if you PUT it in the other toilet. However, GET IN TOILET still works.
17. Although you can GET IN WINDOW SEAT, you're never truly in it. Anything you DROP there lands in Morning Room, you can GET and otherwise interact with anything in Morning Room, but not anything in the closed window seat.
18. If you KNOCK ON a DOOR with someone on the other side you get, "Someone shouts, "Go away!"". This even works if you KNOCK ON FRONT DOOR at Entry Hall when someone's on the front porch. This also works in a few places without anyone there--for example, if you KNOCK ON DOOR inside West Bath, or KNOCK ON GARAGE DOOR. Oddly, it also happens if you KNOCK ON BMW or MERCEDES.
19. One object that's present everywhere is Alicia's car. If you use the word CAR anywhere except Garage or Circular Driveway (when the police car is there), then the game defaults to Alicia's car, which is supposedly "parked back near the barn". Most verbs used on CAR give normal failure messages, as if it were in the room, always calling it "the Alicia's car" or "a Alicia's car". EXAMINE CAR says, "You can't see the Alicia's car here.", but this is almost the only verb that claims the car isn't there.
20. If you're carrying the broken glass, you can DROP LIQUID or THROW LIQUID, but it doesn't show in the room description. After you do this, you can PUT things IN GLASS.
21. If you pick up the waste basket then DROP CARD or THROW CARD before looking in it, then the business card is permanently invisible and untakeable.
22. If you're carrying Veronica's body, you can DROP or THROW STAIN, or a noun representing her "vital signs", such as PULSE or BREATH. These objects don't show in the room description, and the "vital signs" disappears if you DROP BODY in a different room.
23. EAT BODY or READ BODY picks up Veronica's body without revealing the bullet.
24. At Ballroom, Near Fireplace, you can use the syntax BURN object IN FIREPLACE. The game won't check on if you're carrying the object, so you can put quite a few things, including people and abstract concepts, in the fire. --Torbjörn Andersson
25. GET something FROM FLOOR says, "The room isn't here."
26. When you get to the office, most verbs other than GET treat the lariat as ambiguous. For example, EXAMINE LARIAT says, "Which lariat do you mean, the lariat or the lariat (no, you left it with your coat)?" This stops after you pick it up, or refer to LARIAT or ALL in the lariat's absence.
27. Some commands treat Veronica's body as a person and give inappropriate default responses. For example, GIVE something TO BODY says, "Veronica's body refuses your offer.", and ACCUSE BODY says, "Veronica's body shrugs off your accusation." GOODBYE BODY says, "Don't tell me you're leaving already!", unless you're carrying it, in which case you're told, "Veronica's body isn't here."
28. The sofa is takeable. You can leave it elsewhere but go back to Living Room, Sitting Room, or Media Room and pick it up. You can SIT ON SOFA while you're carrying it, after which EXAMINE SOFA locks up. You can also put the sofa in the hamper. If the sofa is in the closed hamper and you're in one of these sofa rooms, then EXAMINE SOFA says, "You can't see the sofa here.", GET SOFA says, "You can't reach that.", but SIT ON SOFA works.
29. The label in the wet overcoat is untakeable, but if you're holding the coat you can DROP or THROW LABEL.
30. The GO TO command can give you more information than you should have about the whereabouts of objects and people. A successful GO TO something tells you what room you're headed to, but doesn't check that you know where the object is. So for example, GO TO COMPUTER responds, "(On your way to the Office.)". To get this to work with a person you can't see, you must first use them as an object, then use GO TO HIM or HER. You can track the location of almost anyone this way.
31. Anytime after your encounter with Smythe and the party guest in the devil costume is over, the guest has vanished, but GO TO THE DEVIL says, "You already are."
32. You can FIND objects you can't see. For example, when you first get to Office, FIND AGREEMENT says, "It's right here."
33. After Veronica's death, FIND BODY says, "I don't remember where Veronica's body is.", even if it's right there.
34. If you drop the bullet from the gunbelt, it stays invisible. --Torbjörn Andersson
35. There is no output if you BURN something flammable IN something besides the fireplace. --Torbjörn Andersson
36. Trying to DESTROY someone prints garbage: DESTROY BARTENDER says, "Trying to destroyJohnson with a ck it's k feher x is silly." --Torbjörn Andersson
37. If you take Veronica's body, then LOOK BEHIND LARGE DESK says, "There's nothing behind the large desk.", but if the ambulance attendants take it, LOOK BEHIND LARGE DESK says, "You can't see Veronica's body here." --Torbjörn Andersson
38. SHOW WESTERN MASK TO ALICIA says, "That looks like Veronica's mask." --Torbjörn Andersson
39. If the western mask is not in a closed container or the fireplace, UNMASK picks it up, even in another room. --Torbjörn Andersson
40. If you KILL a door, it is broken and can no longer be opened or closed, but if closed you can go through it, opening it. --Torbjörn Andersson