After the girl throws her boyfriend's high-school-ring necklace into the lake, he starts driving away and the necklace is around the girl's neck, then it disappears again.
When the 1972 Pontiac LeMans is being driven before it catches fire it has one set of rims (originals) and when it's blown up the rims are clearly a different type, indicating that two vehicles are used.
In the scene where a character is speeding along a road and sees something that causes him to brake hard, swerve, and skid to a halt, skidmarks from previous takes are visible on the road.
When Hannah says "Do you have anything smaller?" while trying on bathing suits in front of a mirror, her mouth doesn't move.
Crew reflected in the bikers' helmets before the chase begins.
When one of the characters falls into the hole after running through the woods, the "earth" behind her is moving at one point when she backs into it, showing it to be a sheet of material rather than dirt.
When young Angela uses her inhaler, she is on her knees, then when the camera switches back she's sitting on her bottom with her legs out.