Audio problem: At the theatre, when the cook talks to Mark Hamill off-stage he sounds exactly like Moe the bartender.
Continuity mistake: The bible that Reverend Lovejoy returns at the library is bigger in the first shot.
Continuity mistake: Right at the beginning, the first shot of Homer and Lisa shows the door open. After Homer says 'in the most wicked condition' and the shot changes the door is closed.
Continuity mistake: When Lisa is in Principal Skinner's office, there isn't a scoreboard control panel on the desk, yet when there is an overhead shot of his desk, one suddenly appears.
Marge Simpson in 'Screaming Yellow Honkers' - S10-E15
Deliberate mistake: When Marge goes offroading to get home there is a grassy field which leads right up into the Simpson driveway. However, in all other episodes there is no grassy field and instead there is a house in the spot where the field would be.
The Old Man and the C Student - S10-E20
Continuity mistake: Springy the Olympic spring that Homer makes has 9 coils. Then we see the large version that has many coils, at the Olympics meeting. Homer's original spring is in his hand and now it has many coils also. When it cuts back to the stage, the large spring has few coils, like the very first time we saw Springy.
Thirty Minutes Over Tokyo - S10-E23
Continuity mistake: Marge loses one of her shoes in the "lava" , but when they fall down, Marge wears two shoes again.
Wild Barts Can't Be Broken - S10-E11
Continuity mistake: The Springfield Isotopes win the championship at Springfield Stadium and it's daytime. But Homer and the guys watch the game live on TV at Moe's Tavern, and it is nighttime.
Sunday, Cruddy Sunday - S10-E12
Continuity mistake: When Homer tells Lisa who is going with him to the Super Bowl, he says Comic Book Guy is going. In the shot showing the people lining up, Comic Book Guy is not there. He then appears on the bus later on.
Continuity mistake: When Bart says "Everybody, come quick!", the fridge door goes from open to closed.
Thirty Minutes Over Tokyo - S10-E23
Continuity mistake: When Homer breaks into Flanders's house there's a shot of the refrigerator and nothing is on it. In a closer shot, there are tickets being held up by a magnet.
The Old Man and the C Student - S10-E20
Continuity mistake: The springs on Homer's hat change size when Agnes answers the door to him.
The Old Man and the C Student - S10-E20
Continuity mistake: When Homer offers Lenny some nuts a spring pops out and gets Lenny in the left eye, but the next time we see Lenny it is in his right eye.
Continuity mistake: After Homer falls asleep at the wheel the truck starts to turn the corner. As its going around the bend you can see a white line on the road. The following shot the line has disappeared.
Continuity mistake: In this episode and maybe in other one, the door to the basement is at the entrance of the house, but that's a closet in other episode.
Tree House of Horror IX - S10-E4
Audio problem: At the end when Bart says 'Don't forget Ken Star' his mouth doesn't move.
Tree House of Horror IX - S10-E4
Continuity mistake: In "Starship Poopers," Kang always wears a glass thing on his head, but whenever Homer beats him up, it's gone.
Revealing mistake: After Apu and Manjula go by in horse and cart everyone returns into Moe's. When they walk in they just disappear into a brown wall which is where the inside of Moe's should be seen.
Audio problem: In the beginning, we are at the power plant. We find out that Burns has packed his own lunch, and one thing he packed is a bullion cube. His mouth doesn't move right when he says "Bullion cube".
Continuity mistake: When Homer is talking to the makers of Police Cops, the person on the far left's hair changes from brown to grey instantly between shots.
Answer: I'm not sure if this is the one you're thinking of, but an episode of "Ray Bradbury Theatre," called "A Sound of Thunder," dealt with a similar matter: a group of hunters travel back in time to hunt dinosaurs, only to find things have changed when they get back because someone stepped on a butterfly.
Xofer