When Pugsley blows up a toy train, Mr. Hilliard can be seen for just a second standing next to Gomez, in stock footage from The Addams Family Goes to School (1964).
After Morticia clobbers Uncle Fester on the head, thinking it's Gomez, she bends down and her spider tassels are missing from the bottom of her dress.
The threads moving Cleopatra are momentarily visible.