When Thing snatches the mail from Mr. Briggs, he is a left hand instead of his usual right.
When Lurch is in the doorway to Fester's room and asks 'You thumped?' the shot is grainy and slightly out of focus. This is because it is the same as the previous wide shot, but zoomed in on Ted Cassidy.
When Fester and Gomez mention Jack LaLanne's name, their voices are dubbed because they're saying a different name. The different lip movements are noticeable.