Revealing mistake: In the part in the motel where THING is doing morse code on the table to Gomez you can see the cut-out in the table where the guy's hand is sticking through the table.
Continuity mistake: During the Mamoshka dance, they cut away to Tully and Greta talking, behind them walking around we see guests who are in the ballroom at the same time.
Continuity mistake: When Mrs. Craven introduces Tully to her "son" Gordon, Gordon grabs Tully by the shirt collar with his right hand but in the next shot is using his left hand.
Continuity mistake: During the play, when Pugsley slices Wednesdays throat, there is blood on the front of her shirt. When she turns to face the audience, the front of her shirt is clean.
Deliberate mistake: At the beginning, the carolers are on the porch singing. The camera never pans, so this means that the carolers are facing away from the house (towards Gate) - seems rather odd. The only reasons for this would have been to make the clean camera move through the crowd and up the house.
Revealing mistake: During the Mamoshka dance, right when Gomez throws the knife high in the air (the one that lands in Fester's mouth), the camera goes to a wide shot when the knife is in the air, and you can plainly see that the man juggling with Fester isn't Raul Julia, but his stunt double.
Continuity mistake: When Fester comes down to the party and the shot cuts to everyone stopping dancing when Morticia calls for their attention, close to the center of the room is a woman with very frizzy orange hair dancing with a man with lots of facial hair. When the shot cuts back to Fester and Morticia, this couple is right behind Fester near the door to the hall.