This movie takes place in 1983, yet the taxi cab that Abby gets into near the end of the movie appears to be a 1986 model. The car has a high-mount brake light, which didn't appear on cars in the U.S. until the 1986 model year.
When Kenny, Mark and Donald first confront Owen at the ice pond, Mark's hands are out of his jacket. When the camera angles to show Kenny, Mark and Donald's faces, Mark's hands are suddenly in his jacket.
The goof items below may give away important plot points.
When the Policeman is breaking down the door to enter Abby's apartment there is quite a bit of damage done to the door, especially around the area of the door handle, but as he enters the apartment with his gun raised the door appears to have no damage at all.
When the disfigured Thomas leans forward to Abby on the hospital window ledge, the camera angle shows he is a mere inches from Abby - as if they were about to kiss goodbye. When the camera angle switches, they are almost arm's length apart.
When the Policeman breaks down the door to Abby's house, he is holding a 1911-type pistol. This sort of weapon is single-action, which means the hammer must be cocked before it can fire. The hammer is in the forward/down position the whole time, meaning he was entering what he knew to be a dangerous situation with a weapon that would not fire if he pulled the trigger.