In the final escape scene, Roger and Judy scale the high wall by grabbing the barbed wire running along it. When Roger falls over the wall, we see no visible means of scaling the wall from the outer side, yet he is somehow able to climb back up to the top to rescue Judy.
When possessed Frannie and Max come out of the coffin they're both naked, but at the end of the movie they're wearing clothes again. Since they're possessed, it makes no sense for them to put on their clothes.
Jay is killed while having sex with Suzanne (shirt's open and pants down) but when Judy and Sal discover his body, and when he's later possessed, he appears fully dressed and his shirt is buttoned up.
In Italian version at the beginning of the movie the characters talk about a party for "Carnevale," while later in the movie when they're going inside Hull House, they talk about "Halloween" or "Ognisanti."
In the convenience store, the repeated sounds from the arcade machine are actually from the Atari 2600 home version of Pac-Man (1980).
In the opening scene when Stooge, Helen and Rodger are in the car Stooge asks Helen to take the wheel. When he does he accidentally calls Helen "Stooge."
In the hallway scene where Angela is floating after Judy, her mouth movements don't match the timing of the demon moans.