When Annie is on a couch in Diane's house and remarks that the photos are old, the lighting on her changed from previous shot.
A student is stabbed underwater. It's doubtful it would be possible to throw a javelin with that strength and speed underwater to kill somebody with efficiency as shown in the movie.
The girl is impaled by a javelin to the wall. It literally carries her and nails her to the wall, but in reality the javelin would have to be thrown/shot by a very big force to do that.
A man in the movie breaks down a padlock with a one hit of blunt object. It would require far more force than that to break it.
A doctor supposedly pins down a living bug, but he clearly misses it.
A javelin thrower is killed by the killer which impales him with a thrown javelin. Problem is the killer is shown far away on a much higher position than him and not only that would be incredibly hard to hit, but it wouldn't impale him like shown.
A man is killed by a thrown javelin on a staircase. The staircase is obviously too narrow to wind up a good throw with a javelin as big as that.
The camera's shadow (notably the 400' MAG) is visible on the ground, filming the POV shot of the killer stalking the pool.