Revealing mistake: When Homer says "Oh, baby we got him now" whilst arriving in the airport, you can see half of Doctor Hibbert's face in the background, but you can see he doesn't have any eyebrows or eyes.
Revealing mistake: At the U.N. meeting, Bart places his glasses down on the table to read out the Libyan exports. As he puts the glasses down, you can see they are unrealistically floating in the air.
Mypods and Boomsticks - S20-E7
Revealing mistake: When Bashir leaves the Simpson house, he doesn't have any eyebrows. (00:12:40)
Brother Can You Spare Two Dimes? - S3-E24
Revealing mistake: When Herb reads the article about Homer winning the $2,000 the writing on the newspaper (except the headline) is just gibberish. (00:09:20)
Revealing mistake: At the craft store when Homer and Bart go in, the door opens before Homer pulls the handle. (00:14:00)
Itchy and Scratchy: The Movie - S4-E6
Revealing mistake: When Nelson blinks right before Milhouse says "No one who saw the movie would say that", Nelson's right eyelid is white for some reason. (00:18:20)
Revealing mistake: When Nelson is offering Lisa the fraction quiz answers, right before Lisa says she's never cheated before and won't again, her right hand disappears for a single frame.
Marge vs. the Monorail - S4-E12
Revealing mistake: When Homer is driving along and singing, he passes three houses in the background. Those exact same houses reappear three times in the same shot even though he never turned around. (00:01:15)
Who Shot Mr. Burns? (1) - S6-E25
Revealing mistake: Right at the beginning, watch as Principle Skinner enters the classroom, there's a clock on the top left hand of the screen. The majority of the left side of the clock is not drawn in.
Little Girl in the Big Ten - S13-E20
Revealing mistake: When Bart says "I don't feel so good" after being bitten by a mosquito, his lower body under the table is not visible.
Revealing mistake: There are two Ruth Powers in the same shot, when the people are crowding on Homer at the mall.
Don't Fear The Roofer - S16-E16
Revealing mistake: When Homer sits on Lenny's cake in Moe's bar, some of the frosting appears to stay in mid air, and when the frosting is on the wall, it doesn't slide to the ground like frosting would.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.