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.Carl Missouri
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.
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.