Disappearing Soup
When Ethel is making the stew right before Junior is killed, the level of soup changes depending on whether the shot is in front of her or behind her.
Magic Sweater
When Pam runs through the woods, her sweater magically disappears and re-appears in shots later.
Brave Photographer
When the cop is explaining to Pam that Roy is the killer, he shows her some newspaper clippings. One of them has a photograph of Jason with his hockey mask on. What brave photographer took that picture?