Pee-wee's Playhouse (1986)

Monster in the Playhouse - S1-E9

Revealing mistake: As Reba the Mail Person exits the Playhouse, she goes past the window, and you can see her exit the sound stage through a door painted the same as the background.

Movie Nut

Pee-wee: You all remember what to do whenever anybody says the secret word right?
All: Scream.
Pee-wee: That's right. For the rest of the day, whenever anybody says the secret word, scream real loud. Ready? Let's try it.