Continuity mistake: During the big song near the end, there is a birds eye view of everybody outside dancing and singing, and Krusty, Chief Wiggum and Skinner are there outside with everyone. But in the very next shot, they all appear out one of the windows, singing a line to the song.
Simpsons Spin-off Showcase - S8-E24
Continuity mistake: In the beaver sketch, "Lisa" is holding Maggie in front of her to show everyone her first tooth. In the next shot, Maggie is instantly to her side.
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: The vehicle they are driving changes form when they drop off Bart at military school to when they drive home. They leave with Homer's purple car (obvious from the crumpled headlight) and as they leave the military school it's Marge's orange car (obvious from the ski racks and lack of a trunk). It's clear from the entire scene that this is all one trip.
Continuity mistake: When the mob is at the gates of Belle's house, the door bell is first seen on the left hand side of the gates. In the next shot (behind Flanders) it is in the center of the gates.
Continuity mistake: At The Town hall meeting Mayor Quimby's microphone is first seen to be plain grey. In the following shot it has dark lines on it.
Lisa's Date with Density - S8-E7
Continuity mistake: The hole in Nelson's locker increases in size.
Continuity mistake: When Bart climbs on the roof to get the remote control plane, he falls, and smashes into a gargoyle which falls and smashes beneath him. But when he is hanging from the roof, below him the gargoyle is nowhere to be seen.
Continuity mistake: When Helen and Reverend Lovejoy are singing to Belle, in the big song, Principal Skinner and Apu are standing behind them. In the next shot, they've disappeared.
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Thing and I' when Homer and Marge return from driving around in the rain, there is no doorknocker on the door. Then in the shot where Hugo escapes, we see the door open with red scratches on it, and there is a doorknocker. Then later when everyone leaves to find Hugo, it's gone again. (00:02:40 - 00:04:55)
Other mistake: The doctor in the insane asylum shows a video that shows a child Ned Flanders from 30 years ago. Ned is 60.
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Thing and I' when Lisa and Bart are questioning Homer about the 'thing' up in the attic at breakfast, Homer is the only one eating. But in the next shot you can see Lisa and Bart with breakfast too. (00:01:30)
Homer vs. the 18th Amendment - S8-E18
Continuity mistake: When Homer and Bart are escaping from the cops after stealing kegs of beer from the dump, they drive through a cemetery and slam into a headstone, breaking the right headlight of the car. When they pull into the driveway back at Evergreen Terrace, the left headlight is the broken one.
The Twisted World of Marge Simpson - S8-E11
Continuity mistake: At the fair, Helen Lovejoy picks up a pita and bites a corner off it. There are a few angle changes and when it goes back to her and you can see her pita looks untouched. It's unlikely she just picked up another one or finished a whole one in that time.
Factual error: You wouldn't hear a dial tone on one side of the phone line if the phone on the other side was cut from the receiver. You wouldn't hear anything.
Continuity mistake: Ned's clothes are cut and torn in the huricane, but the next day, they are somehow clean and intact. He can't have gone home and changed, otherwise he would have seen everyone repairing his house.
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Thing and I' Dr. Hibbert is about to explain Bart's irregular birth. In the closeup before the story, Dr. Hibbert has a stethoscope around his neck. This wasn't there in previous shots. (00:03:50)
Continuity mistake: When handing out the mail, Marge places a miniature jerry can of petrol on the table. It is gone in the next shot.
The Secret War of Lisa Simpson - S8-E25
Factual error: When Bart smashes all the glass in Springfield(as a result from the sound wave he made by speaking through the mega phones)the car windows get smashed as well. But when we see Chief Wiggum and Homer's car, the glass is still intact.
Continuity mistake: Marge said to Bart she would make Butterscotch chicken for him, but in the Season 2 episode "Blood Feud" Marge says Bart's allergic to butterscotch.
Brother From Another Series - S8-E16
Continuity mistake: In the scene where Sideshow Bob and his brother Cecil are arrested and sits in the the back of the police car, Cecil is missing when Sideshow Bob is talking to the people outside the car though his window. They both entered the car and they can both be seen when the car leaves.
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