After Ricky attacks the kid in the theater, he goes back to sit next to Jennifer. She and Chip don't see him return and he sits in the seat to her right which is next to the wall. There is no way that he could have gotten to the seat without them noticing him.
After Ricky shoots the car's engine and it explodes, there's no one inside the car.
The car changes position after it capsizes and explodes.
Though doctor wipes sweat from his face, some of it remains above his lip, and that sweat disappears right after, when Ricky comes to him after the talk about the murder with umbrella.
While killers in horror films often have superhuman strength, pushing an entire umbrella through someone would need the force of a machine. Coupled with this, the umbrella would also break, making the scene depicted with Ricky killing someone impossible.
The way the police officer confronts Ricky after he murders Jennifer is so poorly done. He simply approaches Ricky while pointing his revolver at him without giving him the usual proper instructions, giving him the opportunity to grab his revolver and turn it against him. In reality, a police officer would order the suspect to put his hands up or on top of his head so that the officer would know that the suspect is unarmed and that they wouldn't be able to make any sudden movements while the officer attempts to apprehend them.
During the flashback sequence with the couple being caught by Mother Superior, they are a different couple than the actors in the first film. For whatever reason of not using the exact same material, these inserts were reshot again for the second film.
Obvious stunt double for Ricky when the car flips over and catches fire. The double has bigger hair.
After Ricky kills a man with an umbrella, rain first starts to fall on one exact spot but then it spreads out, so we know it's not real rain.
When Ricky discusses his foster family, he claims he moved in with them when he was 12 years old and stayed until his stepfather died five years later. We see him standing in front of his grave, but Ricky appears to be a grown adult, not a 17-year-old. The problem persists throughout the film, despite the fact that he is meant to be a teenager.
When younger Ricky kills the man who forced himself onto a woman, it happens years before the present day yet the Jeep used is a brand-new (at the time) '87 Wrangler.
In the VHS version, when Ricky crosses the street during his shooting rampage, the entire crew is visibly standing around, equipment and all. The DVD release crops out most of the crew, though a boom mic and the top of someone's head are still visible.
As Ricky recounts the shooting of Santa on Christmas morning 1984, he claims the janitor was wearing the outfit when he was shot down, mistaken for Billy. This is a contradiction because in the first Silent Night, the pastor, Father O'Brien, was actually wearing the suit.
Towards the beginning when Ricky says Mother Superior got him the job at the toy store, it was really Sister Margaret in the first film.
When Ricky enters the room behind Mother Superior, she turns too fast like she's already facing his direction.