Continuity mistake: When Homer is in his mansion watching Weird Al's parody of his song, the trash on his floor (frappuccino cups, pizza boxes, etc.) keeps switching places.
Continuity mistake: During Sadgasm's first appearance, a girl with a green skirt, a red sleeveless shirt and grey hair moves from place to place during the "concert."
Deliberate mistake: When Homer is watching Weird Al's parody of his song, he never turns the t.v. on and it isn't already on, but somehow it turns on.
Continuity mistake: When Homer lights his albums on fire, he watches Weird Al doing a parody of his song. When it's over, the fire is gone and there are no ashes where it would be.
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.
Sex, Pies and Idiot Scrapes - S20-E1
Continuity mistake: When Homer has Snake cornered, the second rung on the ladder behind them is broken. In the next shot it's not broken, then it is again.
Sex, Pies and Idiot Scrapes - S20-E1
Continuity mistake: During the St. Patrick's Day parade, Bart is wearing a green shirt throughout the whole scene, but he's wearing his usual orange shirt when he says "Where's the IRA when you need them?"
Continuity mistake: In Bart's room, while Lisa is carrying the Krusty phone, the back of the phone Krusty's neck changes from white to yellow multiple times in the same shot.
Continuity mistake: During the 20 years ago flashback, when Maude goes to spray water on Homer's pants, she is sitting on a tree in the background. When Homer and Marge talk, she doesn't spray water on his pants, but in the next shot she is gone. (00:17:35)
Revealing mistake: When Alberto says "If you had completed your seduction of me, I never would have met Sylvia," his shirt collar briefly turns into the same color as his blazer.
Homer and Lisa Exchange Cross Words - S20-E6
Plot hole: Gil would have been disqualified for cheating at the Crossword Championship. (00:15:00)
Mypods and Boomsticks - S20-E7
Revealing mistake: When Bashir leaves the Simpson house, he doesn't have any eyebrows. (00:12:40)
Mypods and Boomsticks - S20-E7
Continuity mistake: When Homer runs into the old Springfield Mall at the demolition, if you look closely you can see Hans Moleman there. However, when the Duff truck crashes and Homer swims to save the beer, Hans Moleman is now in the truck. (00:18:30 - 00:19:05)
The Burns and the Bees - S20-E8
Factual error: When the bees fly to Mr. Burns' stadium, they are shown as seeing the stadium with 11 eyes. Bees only have 5 eyes. (00:18:15)
Continuity mistake: When Marge says good night to Bart he has pyjamas with sleeves on. Then when he puts on Lisa's nightgown (which doesn't have sleeves) to make Marge think he is Lisa, Bart's pyjama sleeves can't be seen under Lisa's nightgown. (00:12:25)
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.