This is my original review, preserved for historical purposes. Updated commentary may be available on my page for the game.
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.