When Libby and Hurley are in the forest with the food Hurley has stolen, he throws ranch dressing all over the place and over his shirt. In the next shot a second later, his shirt is clean before it goes back to dirty again.
When Hurley and Libby are kissing on the cliff, the camera quickly changes angles. On the second switch, her bangs suddenly disappear from near their mouths, but before and after it they are there.
When Leonard plays Connect Four, he drops 3 red discs in the bottom row. Some of them disappear and reappear during the scene.
In the commentary section of the DVD, director Jack Bender brags that he watched other movies to make sure that imaginary characters don't impact the real world (like how Dave doesn't touch the door at the bottom of the stairs). However, while at the top hallway, going down the stairs, and when Dave jumps out of the window, Dave is casting a shadow on the walls.