During one of the last scenes of the movie, a man rips Jennifer's top completely from front but during next few seconds her top is intact but later on when she goes to bathroom her top is ripped again.
When a guy exits his car to give another car a push, position of his girlfriend is not in the same position she was in previous shot and she is now clearly leaning on her seat with her head. These two shots are one after another and are supposed to show continuity.
You can see the "body" underwater move her eyelids in the beginning of the film.
The high-rise apartment building in which Jane resides is shown as being one of a twin-tower structure. However, the still-shots of this complex clearly reveal that it is in fact a single tower but photographed as a composite; this being for the purpose of conveniently allowing Jane to view Stanley through her binoculars in his "twin" apartment building directly opposite hers.
The killer decapitates a man with a cleaver in one hit while the man is in a sitting position. This would require an insane amount of strength and precision. Even the executioners in the old times when people's heads were cut off with a large axe as a punishment sometimes failed to land the killing blow in one hit.
When the killer turns off his TV, a crew member can be seen in the reflection.