Factual error: During the theme, the Simpsons' mailbox flag is on the wrong side of the mailbox. (Flags are always on a mailbox's right side, when viewed from The Front. The one at the Simpson home is on the left side of their mailbox). (00:01:45)
Revealing mistake: In all the intro scenes from season 2 to season 20, episode 9, in the wideshot where Bart skateboards out of the school, if you look closely, part of the sky can be seen on the ground. (This mistake may only apply to the original format).
Factual error: Moe has a double barrelled shotgun that fires four times. A double barrelled shotgun can only fire two shells before having to reload.
Continuity mistake: The notepad Homer draws the blueprint on changes from having content on it to being blank so he can draw on it.
The Simpsons Christmas Special: Simpsons Roasting on an Open Fire - S1-E1
Continuity mistake: When Homer walks out Circus of Values with his purchases, he bumps into Flanders, and all the presents scatter everywhere. Watch out for a red present with a green bow that goes flying near the left hand side of the screen. As soon as it hits the ground it turns into a pink box with a white bow wrapped around it, with no shot changes. (00:10:15)
There's Something About Marrying - S16-E10
Audio problem: Homer is strangling Bart and tells Lisa to hold his spot. You can hear Lisa say, "Why you little," and you can hear Bart choking, but neither of their mouths is moving.
The Simpsons Christmas Special: Simpsons Roasting on an Open Fire - S1-E1
Continuity mistake: Bart yanks Homer's beard off in the mall, Homer then takes Bart into Santa's workshop. When they are inside Santa's workshop, Homer's beard is back on, despite the fact that he let it fall to the ground when Bart pulled it off.
Moaning Lisa - S1-E6
Continuity mistake: Marge drops Lisa off at school for band practice. She walks up to the entrance steps, and then converses with two boys. Then her teacher comes up, and Marge gets annoyed. When she comes to grab Lisa back into the car, Lisa is right by the roadside, along with the two boys and the music teacher.
Treehouse of Horror XVIII - S19-E5
Continuity mistake: When Marge puts the logo Hugh Laurie in the microwave, he splatters in the corner. Next shot the splatter is in the middle of the microwave. (00:01:00)
Other mistake: When a comet is about to hit Springfield, everyone is standing on a hill in the shadow of it. But the shadow is still there even after it breaks up.
Please Homer, Don't Hammer 'Em - S18-E3
Continuity mistake: It is revealed that Bart is very allergic to shrimp. However, in "The Frying Game" (where Marge and Homer get "framed" for a murder and end up on "Frame Up"), Bart says he was in the green room and that he "ate so much shrimp."
Factual error: The cover of "World's Finest Comics" #94 is shown on the wall at the comic book store. It features Superman with a sign that says, "Gotham or Bust," and Batman gulping. The actual cover of "World's Finest Comics" #94 shows Superman being sprayed with liquid kryptonite while Batman and Robin catch sight of him.
Continuity mistake: On various occasions The Front door, viewed from the inside of the house, has hinges - and therefore opens - on the wrong side, if you compare it to the shots of it from the outside.
Audio problem: During Marge and Homer's argument at the beginning, when Homer says that Ned Flanders is a jerk, his mouth doesn't match what he's saying. (00:01:55)
Continuity mistake: When Martin does his book report, on the blackboard it says 'An afternoon with Ernest Hemingway: Reflections by Martin.' But when Bart goes up to do his report, the blackboard is clean. (00:01:25 - 00:02:20)
The Simpsons Christmas Special: Simpsons Roasting on an Open Fire - S1-E1
Revealing mistake: When Marge is speaking on the phone to her sisters, her sisters say something that sounds like they said that Homer is ugly. Marge then puts down the phone wire that she was curling round her finger. You can temporarily see that the phone wire disappears when Marge puts it down. It is at the very end of an early scene, because as soon as this happens it changes to another scene. (00:04:40)
Factual error: After Bart's prank call with Moe, you can see Bart and Lisa laughing. However the telephone's wire does not connect to the receiver. (00:09:40)
The Last Temptation of Homer - S5-E9
Continuity mistake: Mr Smithers throws a notepad at the person saying that a real emergency exit is required, but when we see it on the floor it is now a book with a green cover.
Continuity mistake: Near the start, when Homer is going through the medicine cabinet and we see various items. He takes out various hair growth products, and then when we see what's left, the floss has vanished. (00:01:10 - 00:02:20)
Tree House of Horror XII - S13-E1
Audio problem: In 'Wiz Kids,' when Edna asks Harry Potter if he is chewing gum, he replies "No ma'am, it's brimstone," however it's clear that a different line was originally intended. (00:15:25)
Suggested correction: I just rewatched this scene and his mouth visuals match with what he's saying; no mistake here.
