Hitchhiker's Guide to the Galaxy

1. In version 47, you can get the plotter out of the case using READ PLOTTER, LOCK something WITH PLOTTER, or BREAK something WITH PLOTTER. Now it's in your inventory.
2. In version 47, when you're sitting in the pilot's seat aboard the presidential speedboat, anything you DROP lands on the seat. It still shows in the room description, but you can no longer interact with it.
3. In version 47, PUT PEANUTS IN a container says, "How can you do that?".
4. In version 47, you can EAT SANDWICH without having bought it.
5. In version 47, if you DROP TEA in Access Space, then it stays there instead of disappearing.
6. In version 47, you can PLUG either plug IN something it's already plugged in.
7. In version 47, if you crash into the cliffs as Zaphod after opening the toolbox, then come back, the toolbox is still open.
8. In version 47, PLUG or PUT DRIVE IN a receptacle or ATTACH DRIVE says, "In case you hadn't noticed, there are two connections leading from the spare Improbability Drive...", even if you've already burned away the large plug.
9. In version 47, if you give the guard a command in Vogon Hold, you can understand his reply of, "Resistance is useless!", without the babel fish.
10. In versions 47 and 56, you can get the plotter out of the case using HANG PLOTTER ON HOOK. You get the response, "It slips off the hook." Now it's sitting on the deck, where you can pick it up.
11. In versions 47 and 56, PICK UP is treated differently from GET. In particular, PICK UP ALL tries to take numerous untakeable scenery objects that are ignored by GET ALL, such as the "name" object.
12. In versions 47 through 58, while you are Zaphod, WHERE IS PHIL crashes the interpreter. The Infocom interpreter exits with no error message, but Frotz says, "Fatal error: Stack overflow".
13. In versions 47 through 59, if you leave your dressing gown on as Arthur, then while you are someone else, GET UNDRESSED will take it off and say, "Okay, you're no longer wearing your gown.", failing to recognize that you're not Arthur at the moment.
14. In versions 47 through 59, as Zaphod at Dais, if you say GUARDS, DON'T DROP YOUR WEAPONS, they do. Some sort of reverse psychology, I guess.
15. In versions 47 through 59, GET ME FROM FORD says, "That's easy for you to say since you don't even have Ford Prefect.", or similarly for other people.
16. In the Solid Gold version, you can interact with the bulldozer driver in several extraterrestrial locations, including Captain's Quarters, Bridge, and Engine Room.
17. In versions 56 and up, if you die as Trillian while carrying the tweezers, they reappear in Arthur's inventory. --Torbjörn Andersson
18. In all versions, while aboard the presidential speedboat, you can PUT body parts IN WATER. The response is, "Glub...", and your body part disappears.
19. In all versions, you can use the satchel to disconnect the "long, dangly bit" from the atomic vector plotter. With the satchel on the floor, just PUT BIT ON SATCHEL THEN GET SATCHEL. Now the bit is there on the floor, and won't move when you remove the plotter. If you try to GET BIT you get, "You can't -- the long, dangly bit is an integral part of the [name of the room]." PUT BIT ON SATCHEL even works before you've opened the case. You can do the same with the large and small plugs from the spare Drive.
20. In all versions, you can use the thing your aunt gave you to separate the cup, plotter, and/or drive without disconnecting them. For example, if they're all properly connected and on the floor, and the thing is also there, then you can PUT PLOTTER IN THING THEN PUT BIT IN CUP and then go somewhere else and wait. As always, the thing will come to you after a while, with the plotter in it. If you GET PLOTTER THEN DROP IT, the room description shows that it's still connected to the drive and the bit is dangling in the cup--even though those things are in another room.
21. In all versions, if you PUT something ON SATCHEL in Vogon Hold after getting the babel fish, and leave it there, then when Ford leaves the satchel on Bridge, the room description tells you the object is on the satchel, but it isn't actually present until you GET SATCHEL.
22. In all versions, if you PUT something ON SATCHEL THEN READ SATCHEL, you pick up the satchel without removing the object from it. If you go somewhere else and DROP SATCHEL, then the room description there tells you what's on the satchel, but that object isn't there unless you GET SATCHEL. It's still in the room where you got the satchel. It doesn't show in the room description there, but you can interact with it normally, even GET it.
23. In all versions, if you haven't taken the dressing gown yet, then WEAR SLEEVE picks up and puts on the gown, but it's invisible. If you cure your headache while the gown is invisible, then dropping it and picking it up won't make it visible.
24. In all versions, if you DROP TEA in Maze, then it stays there instead of disappearing.
25. In all versions, in Vogon Hold, when you have the babel fish you can BLOCK PANEL WITH FISH, although the game stops you from getting rid of it in other ways. Attempts to pick it back up are treated as attempts to remove it from your ear.
26. In all versions, you can't exit your house with the door closed, but you can enter it.
27. In all versions, you can BRUSH TEETH when you're holding the toothbrush inside a closed container.
28. In all versions, PUT PLOTTER IN CASE claims, "There's no room.", although you just took the plotter out of the case!
29. In all versions, if you're wearing your dressing gown, TIE GOWN says, "You'll have to remove it first." If you then say REMOVE IT you get, "You are already wearing it."
30. In all versons, LOOK IN CIRCUIT BOARD gives no output. --Torbjörn Andersson
31. In all versions, it is possible to take the gown, empty the pocket, drop the gown and pick it up again before you run out of time. It will still claim there's something in the pocket. --Torbjörn Andersson
32. In all versions, if you STAND BEFORE BULLDOZER, the game will tell you that you are "getting close" even if you're already lying in front of it. --Torbjörn Andersson