Continuity mistake: The size of the smiley face on Krabappel's mug keeps changing between shots. (00:04:50)
Continuity mistake: While Edna is holding the "Goddess Bar", her shirt sleeves are pink like her pyjamas, but in the next shot, she is wearing her usual green outfit.
Continuity mistake: Barney is simultaneously seen at the wrestling stadium watching the fight, and at Moe's.
Rednecks and Broomsticks - S21-E7
Continuity mistake: When the Simpsons are on the highway, Apu's car is seen, and some of the octuplets have yellow skin and blonde hair.
Continuity mistake: When Carl and Lenny are lowering Homer onto the train window, Lenny's five o'clock shadow is missing.
Once Upon a Time in Springfield - S21-E10
Continuity mistake: The merchandise Krusty asks the kids to hand back doesn't appear in the kids' hands until the merchandise is taken off them. (00:01:55)
Once Upon a Time in Springfield - S21-E10
Continuity mistake: Mr. Burns shows a box of doughnuts coloured pink, brown and light brown. When the guys grab them they're all pink.
Million Dollar Maybe - S21-E11
Continuity mistake: When the bulls are stampeding, people who were at the top of the stadium and safe can be seen running in front of the bulls.
Plot hole: In a flashback, Agnes Skinner blames Seymour for costing her a gold medal in the 1952 Olympics by kicking her while in her womb. This couldn't have been that Seymour Skinner since he was revealed to be an imposter earlier in the series. It would have been the other Seymour who returned from Vietnam who cost her the gold medal.
Continuity mistake: When Mr. Burns is confronting Homer about his missing slave, Homer's donut is half eaten. During the next scene the donut has a smaller bite taken out of it. (00:14:00)
Factual error: In the scene where Lisa's great great grandmother, and the slave Virgil escape to Canada, they enter Canada across a suspension bridge, which has, on the Canadian side, the Canadian Maple Leaf flag. In 1860, Canada was not yet even a country, and the flag was the Union Jack and the Canadian Ensign. The Maple Leaf was not adopted as the Canadian flag until February 15, 1965.
Postcards from the Wedge - S21-E14
Continuity mistake: When Lisa is talking to Bart in her room, Lisa's necklace is red rather than white when Bart shouts "End all life on this planet!" (00:08:05)
Continuity mistake: When Scratchy's head gets sucked into the dam pipe, the back of his head goes in first, but when he is shot out, he is facing forward.
American History X-cellent - S21-E17
Continuity mistake: When Homer, Carl and Lenny are drinking at Moe's Tavern, Smithers' bottle in front of them on the table changes position between shots.
American History X-cellent - S21-E17
Continuity mistake: When Smithers walks up the desk in his office, there is a box of items in The Front of his desk. But when he is looking at a portrait of Mr Burns, the box is now on the side of the desk.
American History X-cellent - S21-E17
Continuity mistake: When Marge is speaking to Bart, Homer comes out of the house with a pumpkin and you can see there is nothing on the doorstep. But when Marge goes to the car, a green doormat has suddenly appeared, and when she leaves, it has gone again.
American History X-cellent - S21-E17
Continuity mistake: When the policeman is speaking to Mr Burns about the stolen artwork, the molding on the door behind Mr Burns changes between shots.
American History X-cellent - S21-E17
Continuity mistake: When Homer (dressed in a knight's costume) is about to hit a book with his weapon at Mr Burns' place, there is a wine bottle beside him that disappears in the next shot.
American History X-cellent - S21-E17
Continuity mistake: When Homer is showing Carl and Lenny the large bottle of wine he has found in the wine cellar, you can see there is nothing on the shield on the label, but when see Homer holding the bottle a few seconds later, the shield has now two lines on it.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.