When Marty and Claude steal the money during the hide-and-seek game, some of the bills stick out of the top of the bag. By the time they reach the door about one second later, the bag is closed neatly and nothing is sticking out.
When Morticia mixes the punch, she first empties the vial of clear liquid into the bowl, then the darker one. It then cuts to a close-up of her hands and she is emptying a vial of clear liquid. When it cuts back to a longer shot, both vials have disappeared.
When the robbers raise their heads out of the crab-bobbing tub, their faces should be sopping wet and dripping, but they are barely damp.
After Fester turns off the lights for the poem reading, he sticks a bulb in his mouth for light. The area around him lights up slightly before the bulb enters his mouth.
When Thing hands Wednesday and Gomez the mail, the table cloth is moving, indicating a person underneath.
Around 12 minutes into the episode, what appears to be the shadow of the boom mic, in addition to another shadow, is clearly visible on the jackets of the two visitors.
When Gomez introduces the robbers to Fester, he gestures to Claude when he says Marty's name, and vice versa.