Continuity mistake: When Sadgasm is making their first appearance, Homer says, "We are Sadgasm," and there are a few students walking around, but when he says, "And this song is called 'Politically INcorrect'," there is a big crowd in front of them with no time to form one.
Continuity mistake: This episode rewrites the entire backstory of Marge and Homer. For starters, Lisa and Bart were both not born yet, despite the fact that in the episode "Lisa's First Word," the '84 Summer Olympic games were being broadcast, and Bart and Lisa were both there. In this episode, Homer and Marge were just dating, but again in "Lisa's First Word," they were married and living in their Evergreen Terrace house. Additionally, Maggie was born in the episode "And Maggie Makes Three." Homer also lost all his hair in that episode, yet in this episode in the early 90's he still has a full head of hair.
Continuity mistake: When Stefan starts crying at the football, Marge puts some music on and Homer's Sadgasm band appears on the radio. At this point, Stefan's wine glass is in front of the couch on the floor. Stefan then says he needs whale songs - Marge puts an "Oceans of the Sea" CD in the CD player and the wine glass in front of the couch suddenly vanishes.
Love, Springfieldian Style - S19-E12
Continuity mistake: In the third story of the episode, Bart and Nelson throw Martin into the Jukebox despite it never being opened. When he is trapped inside the box the glass is firmly in place as well.
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.