Goofs

The graffiti the teacher writes on the bathroom stall was already there in a previous shot.

At the beginning of the film, the escaped mental patient is referred to as 'William Grayham' and at the end of the movie he is referred to as 'Daniel Grayham'.

When one of the teachers is stabbed to death in the beginning of the movie, the amount of blood on her shirt changes.

Blood from the concealed knife-cross appears on the wall, albeit in an earlier shot had none.

When Mark opens the elevator at the end of the movie, the camera shows that it is empty, yet in the next shot it is clearly not empty.

Doreen gets killed when the killer slices her forehead lengthwise. A forehead cut, even down to the skull, wouldn't be fatal since it lacks vital organs. It would likely result in a scar rather than death, as the area primarily serves as a cushion for the skull. Unless severe blood loss occurs or the skull is breached, such a wound wouldn't be lethal.

There's clearly blood on the back of Julie's shirt after she gets stabbed by the killer and her back is sliced through.

When Mark sits down with Julie she starts talking but there's no dialogue audio. Once the camera cuts back close on the couple, she begins talking again but is synchronized with audio.

The cameraman's shadow can be moving on Cathy's back, during the woods scene.

Cameraman and boom operator/mic reflected in window of Julie's car when she drives off to search Mark's apartment.

During a conversation between Cathy and her friend, the camera and a crew member are reflected on the car window.

For the POV shot of Mrs. Bloom coming up the stairs to Julie's apartment, the cameraman's shadow can be vaguely seen on the wall.

One of Cathy's friends looks out the wrong window to bid Cathy farewell.