Revealing mistake: When Homer is riding Barney like a bull at Moe's, there are duplicates of Sam and Larry (the barflies) in the same shot.
Audio problem: When Marge says "Now that it's empty, who's going to buy their house? What if someone moves in with two Barts? Or four teenage Barts?" her mouth movements don't match most of her dialogue.
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 Carl and Lenny are lowering Homer onto the train window, Lenny's five o'clock shadow is missing.
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: Barney is simultaneously seen at the wrestling stadium watching the fight, and at Moe's.
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: The size of the smiley face on Krabappel's mug keeps changing between shots. (00:04:50)
Plot hole: Warren's (as Bob) face falls off and tries to hold it with his hand. When he appears in the next scene, his face is totally sealed with nothing to seal it with. (00:16:25 - 00:17:55)
Continuity mistake: Bob duct tapes Bart's mouth before telling him his story, but the duct tape disappears when the story finishes. (00:12:30 - 00:14:10)
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)
Audio problem: When Homer says "I need change for a dollar. I also need a dollar", his mouth doesn't match what he's saying. (00:07:25)
Audio problem: When Matt Groening is putting boxes of comic books into the Comics Bongo Group truck, he says "I'm so rich" but his mouth isn't moving. (00:04:40)
Continuity mistake: Just before Apu has a flashback, Reverend Lovejoy is sitting to Apu's left. after the flashback he is sitting to Apu's right.
