Brother From Another Series - S8-E16
Continuity mistake: Reverend Lovejoy is standing behind Sideshow Bob during his speech, but in one shot, he is standing with the protesters, then back behind Bob.
Brother From Another Series - S8-E16
Continuity mistake: While Sideshow Bob is walking out of prison and towards the protesters, Carl's hair is red.
Audio problem: Marge, Bart and Lisa are still heard "oohing" at the plumbing supplies store even after they close their mouths.
Continuity mistake: After Marge bites the candle down to a smaller size, the candle turns back to its original size for a frame.
Continuity mistake: During Bart's stand-up comedy routine, Dr. Nick is seen wearing his usual white lab coat, but in the very next shot of the audience laughing, he is not wearing a coat.
Revealing mistake: While Larry and Homer are talking about Mr. Burns, Larry's drink flickers light pink twice.
Revealing mistake: When Lenny is talking about Homer not needing a degree to work at the power plant, Lenny's five o'clock shadow briefly detaches from his face.
El Viaje Misterioso de Nuestro Homer (a.k.a. The Mysterious Voyage of Our Homer) - S8-E9
Continuity mistake: When Homer is freaking out and starting to hallucinate after eating a few peppers, Jasper is behind a tent stall when Homer is running away from Ned Flanders, but just seconds later when Homer falls to the floor, Jasper is in front of Homer. (00:08:05)
Grade School Confidential - S8-E19
Continuity mistake: Edna Krabappel's name on the envelope is misspelled as "Enda" when Bart is opening and reading the letter, but is spelled correctly when viewed in close-up.
Brother From Another Series - S8-E16
Continuity mistake: Sideshow Bob's seat belt is on in one shot but gone in the next when he has been released from prison.
Other mistake: When Ned Flanders is calling Marge and Reverend Lovejoy from the Leftorium, he says that the three hooligans are in the shop on their motorbikes. But a few minutes before when Ned goes outside the shop to tell the boys to clear off, their bikes are nowhere in sight.
Continuity mistake: When Lisa arrives at Dr. Nick's, amongst the wheelbarrow line is a Professor Frink (and he's not even first in the cue). However, just a few minutes later, when Lisa ends up in the mud pit, we can see Frink amongst the crowd.
Continuity mistake: During The Town hall scene, Ruth Powers (the woman with the red bandana) is seen with blondish hair in two shots. All other shots of her during this scene shows her with her usual brown hair.
Brother From Another Series - S8-E16
Continuity mistake: In the shot where Homer yells, "Marge, I'm going to look for the kids," their door is shut. After Homer sees the wave and runs back inside, their door is open all of a sudden.
Brother From Another Series - S8-E16
Continuity mistake: Ruth Powers' hair is red in one of the shots of the protesters. Her hair is brown in other shots of her.
Simpsoncalifragilisticexpiala(Annoyed Grunt)cious - S8-E13
Audio problem: When Shary Bobbins suggests to Lisa that they could have a tea party on the ceiling, Shary's lips don't match what she's saying.
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