Smoke on the Daughter - S19-E15
Audio problem: When Martin runs with his book when Kirk Van Houten says to Otto "It's for my kid." he shouts "Eureka!" but his mouth says something completely different.
Smoke on the Daughter - S19-E15
Revealing mistake: When Lisa turns on the light while dressed in her Angelica Button costume, her mouth disappears for a single frame.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.