Undo by Neil deMause (credited in the game as "null dogmas") is a 1995 interactive fiction game, entered in the first annual interactive fiction competition. The premise is that when the game opens, you've just finished the last puzzle in a buggy, corrupted interactive fiction game, and have only to reach the exit to win.
The game is very brief--I spent about ten minutes beating it, exploring everything as thoroughly as possible--but it has a few entertaining bits. For example, if you check your inventory, you're told that "You have everything that you need.", and in the Binary Room you can take 0 (or take nothing) and your inventory will change to "You have nothing." If you take other objects, e.g. take 1, then "You have nothing and a 1." You can drop nothing and then "You have a 1 and everything that you need." Inspired by The Hitchhiker's Guide to the Galaxy, if I'm not mistaken, quite appropriate for a piece of IF which is about a piece of IF.
The game's solution more or less makes sense, though you're more likely to stumble across it than to reason it out. I've written some invisiclues-style hints for the game, if you're stuck.
Undo has a few neat ideas and an interesting premise, but it doesn't really do anything with them, and feels more like Speed-IF than a real game. It can safely remain a relic of the past.
In an interview in SPAG #7, deMause explains the final puzzle:
He also explains his purpose in making the game: