When Thing plays chess with Wednesday, he is a left hand instead of his usual right. Thing is also left-handed when serving the puree of aardvark.
When Gomez, Morticia and Lurch examine the freshly-caught boot, the position of Morticia's hand changes between shots.
When Pugsley first leaves the chimp on his bed to go see Fester, he closes the door behind him, but it bounces open again. The next camera angle, focused on the chimp, shows the door firmly closed.