Oh, just saw the thread about a recreation of the A.L.I.V.E. engine
On the demo format:
I don't have Abe's Oddyssee, so, I can only guess, but I think it's the same format.
Demo commands can't crash the game. An invalid command would simply do nothing. I think it's merely an abstraction of keyboard/joypad's state.
However, before the commands, there are some bytes that I don't understand. The first 4-bytes word is always "1", maybe a file-wide identifier, as seen with other formats. Modifying it will lead to a crash. I don't know anything about the 12 bytes following it. I first thought those were commands, but changing them doesn't do anything, it seems.
I think another interesting format is the NxtP format. Tweaking them may show some internals of A.L.I.V.E.