In the flashback sequences when Ricky is talking about the Santa the police accidentally shoot thinking it is Billy, in the original film the Satna was Father O'Brien but Ricky says in the sequel that it was the Janitor.
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.
The car changes angle after turning over and exploding.
After Ricky shoots the cars engine & it explodes, right before the explosion nobody is in the car.
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 movies frequently have super human strength, to push a whole umbrella through someone is really something that would require a force of a machine and umbrella would break because of the force. In other words, it's impossible, and so is the scene showcasing that.
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.
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, you can clearly see that the rain first starts to fall on one exact spot and then spreads out, so we know it's not real rain.
When Ricky talks about his foster family, he says he went to them being 12 years old. Five years later, he tells us his stepfather died and we see him standing in front of his grave. Problem is,it's a grown man and not a 17 year old, problem which continues when he says he is 18 and with that, through the rest of the movie.
In the flashback sequence of the couple making love that is caught by Mother Superior, they are a different couple in Silent Night, Deadly Night Part 2 than the couple in the first film. For whatever reason of not using the material from the first film in the flashback sequence these inserts were reshot again for the 2nd film.
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 retells the shooting of Santa on Christmas morning 1984, he claims that the janitor was in the suit. The priest was really in 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.