When Fester lays down the chalk line, Lurch isn't on the stairs. However, when he is rung for, he is standing on the stairs, with chalk on his legs, as if he'd been there the whole time- which he hasn't!
Gomez: Well, she can't be all bad!
Morticia: Oh, really? Reading Dr. Spock?
Gomez: Oh. I thought it was Dr. Spook!
Fester: It was the only rock in the house!