Throughout the movie, actors can be seen with significantly different outfits and hairstyles.
The beginning of the film was shot during the summer. Trees have leaves. At the end of the movie the trees are dead with no leaves. This is because the filming of the last scene was during the fall/winter.
When Pedro jumps over railing to escape from the killer, this is because he wanted to lure the psycho away from the girls and from Jeff. The reason why he didn't walk down the stairs was because falling from the railing looked more appealing.
Audio corrections were a major difficulty during the post editing phase. Camera shotgun mic muffling sound can be heard in some scenes.
Camera record (red) light can be seen in the reflection of Jeff's hat during the kiss scene. Camera operator Ben Krug can be seen in the reflection of the octagonal window when Sam leaves the house.
The scene of Jeff setting down the camera on the booze table where the Psycho is accidentally caught on film was never added into the official movie. This is a barely visible mistake unless you are familiar with the original screenplay.
Many group character scenes were filmed without all of the actors present on set.