Braid - Changing the recent past a bit in the alternate ending.
"I was screwing around a bit in the last level. Odd, the monsters don't seem to mind walking right off into the spike pit, but apparently they didn't like the idea of coming from up the stairs. I guess the level doesn't have the hints to make them "fall" upward in the appropriate places, since they're not expected to go back here.

Also, because the flash makes it impossible to rewind, I irreparably kill Tim by mistake when I go down to investigate. Whoops!"