When Jerry laughs after confronting Peter and Charley at the top of the stairs you can see metal fillings in his teeth. Even though it's not established how old Jerry is, he's probably from before there were fillings.
In the attack scene in Charley's bedroom, Jerry Dandridge has his hand around Charley's neck, and Charley drives a pencil into the top of his hand. In the next scene, Jerry pulls his hand away and looks at the pencil, and it is sticking more than halfway through his hand. If the pencil has been shoved that far down, it would have pierced Charley halfway through his neck.
In the night club Jerry is walking through the crowd, stalking Amy. At one point he passes a mirrored wall and we see his reflection. Earlier it was shown that Jerry isn't supposed to have a reflection.
In one scene Jerry Dandridge appears in the Brewster house. In the next shot it is revealed that he is standing in Mrs. Brewster's bedroom watching her sleep. However, the first shot was actually filmed in Charley's room. The Ford Shelby Cobra poster on Charley's wall can clearly be seen over Dandridge's shoulder.
When Peter Vincent knocks on Jerry's door, Billy Cole answers it. While Peter's arm is still raised to continue knocking, Billy offers his hand to Peter. Peter lowers his hand, yet in the next shot it is back up in the knocking position.
Before Jerry opens Charley's bedroom window to push Charley out of it, he breaks the window's latch clean off with his free index finger (his other hand is wrapped around Charley's neck). However Charley's Mom thwarts Jerry's plan, and he leaves via another window. In the next scene Charley is seen closing his bedroom window and the latch is back on, which he locks.
When Peter Vincent burns a cross on Evil Ed's head, Evil looks into a mirror before realizing he can't see himself. Just when Ed attacks Peter, you can see the reflection of a crew member and a red ladder in the mirror.
In the opening scene on the television, when Peter Vincent announces his name and goes to impale the vampire with a wooden stake and mallet, his stake is backwards as he rushes in. This was intentional as Roddy McDowell is parodying bad films/actors.
After Jerry Dandridge attacks Charley and runs out of his room, he closes the window that had been nailed shut. After Charley's mother leaves his room, Charley sits down next to the window and we can clearly see that it is open again and the curtains are even blowing in the wind.
Charley is actually closing the hallway window Jerry escaped through, which is right next to his mother's bedroom door. The broken frame from Jerry jamming the door can be seen as they talk, indicating they are in the hallway and not Charley's room. Charley then returns to his room, where his window is still open, after his mother goes back into her room.
This is only noticeable in widescreen versions of the movie. When Jerry is in the club and begins slowly moving towards Amy, for a brief moment, Jerry's shoulder can be seen reflected in one of the mirrors.