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.
Add timeCarl 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.