I accidentally went to an area of that enormous tower in Prince of Persia: Warrior Within that I had previously completed when right at the end of the game. I thought "Well, that's no big deal, I'll just parkour myself out." Wrong.
I jumped, flipped, wall-run, and sliced my way through that mechanical clock-like level, got to the end where you have to jump onto a platform just before the final exit. I wall-run onto the platform perfectly, but the Prince fell through. Thinking it was a mistake, I ran through half the level again to try. Got to the final platform, jumped onto it, and the Prince got his feet stuck in the ground, was running on the spot, and slowly sliding through the platform. Being the determined type, tried one more time. This time I got stuck in the platform yet again, but tired to wall-run up the wall beside it to get myself out of the ground. Nope, this somehow ended up propelling me at about 80mph into the opposite wall, then down the large drop.
It was impossible to get further in the game. I actually shrieked like a banshee when the realisation set in.
Still haven't completed that game.