dtgreene: An Xbox Live game called Shadow Complex originally had a glitch that allowed you to bypass the "end of demo" screen, allowing you to play the entire game. (Of course, I am pretty sure the patched it after the exploit was discovered.)
Related, I have encountered a few games where the copy protection questions can be bypassed without needing any hacks. In Ultima VI, you can use the cheat menu to bypass it by changing certain NPC flags, and Ultima VII has a secret room in the starting town (before the first copy protection questions) that allows you to skip to anywhere in the game, including the final battle.
In Might and Magic IV: Clouds of Xeen, you can use the Mirror to escape the first town and again to get the item that normally requires getting past the other copy protection. In Might and Magic V: Darkside of Xeen, it's even easier: the check only comes when you leave Castleview by the main exit: you can bypass it by leaving through the sewers or (if using characters from Clouds of Xeen) using Town Portal.
It would be funny when a demo has such "exploits" to get out of the demo boundaries and lets you think you play the full game. Then you face another story or later in the game you get a message like: Thank you again for playing the demo. Yes, we knew you can go that far and we hope you had fun. But your journey ends here now. Really. Sorry that we have to be so mean."