Pugsley is said to be 10 years old, even though he was only 8 last week in The Addams Family Goes to School (1964).
When Gomez tosses the curtain-string aside, it is clearly not tied in the hangman's noose that it is shown to be in the subsequent close-up shot.
Pugsley lets Morticia wear his neckerchief, yet in a subsequent scene of him with Gomez, he's wearing the neckerchief again.
When Gomez 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).
When Morticia walks down the stairs in the opening scene, the spider tassels are not attached to the bottom of her dress. They are visible later on when she is relaxing in the playroom.
In the shot of Uncle Fester getting down from the tree, the shadow of the boom mic is visible on the door of the house.