When Detective Winn exits the hotel after failing to find Fenton he announces to the other cops, "We have three dead: one maid and two kids." He left out the dead man found in the trunk of his car (Howard K. Ramsey), whose body he had seen less than an hour earlier.
Despite being one of the few slasher films rated PG13, whenever someone is stabbed, their bodies show no stab wounds when they reappear. The only character with blood stains where they had been stabbed has no holes in their clothing where the stab wounds would be.
When Donna is going back to her room after the alarm has sounded, the camera switches from behind to in front of her. When it does, you can see a man walking around in T-shirt and Jeans. The entire building had already been evacuated and no one should have been upstairs. That and besides the fact that everyone was in prom attire.
After the fire alarm is pulled, Donna rides the elevator to the 3rd floor. When a fire alarm sounds, all elevators go to the first floor and stop working, so no one gets stuck in the elevator during a fire.
The goof items below may give away important plot points.
When the bad guy is shot at the end of the movie, it shows that he was shot in the left shoulder above the pocket on his shirt. The scenes switch from far to close and the bullet wound vanishes and reappears only to vanish and reappear again.