Lisa's Pony - S3-E8
Audio problem: When Lisa rides past on her pony, Ralph is sitting on the fence with another boy who says, 'She really tamed that horse', Ralph says, 'Yes but who can tame her?' However he speaks with a deeper voice as opposed to his higher squeaky voice, and sounds exactly like Nelson Muntz. (00:15:55)
Continuity mistake: The notepad Homer draws the blueprint on changes from having content on it to being blank so he can draw on it.
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 Bart crashes at the social club, his skateboard changes position between shots. (00:05:30)
Lisa's Pony - S3-E8
Continuity mistake: At the very end, the outside of the Kwik-E-Mart is different. (00:21:35)
Other mistake: There are two mistakes when Bart calls the Krusty Hotline: 1, The cereal box says the phone number is '1-909-O-U-KLOWN.' Bart presses eight buttons on the phone. 2, as he is dialing, you can HEAR seven buttons being pressed. (00:01:45)
Continuity mistake: At the end, the barriers outside the tavern change position. (00:21:00)
Burns Verkaufen der Kraftwerk - S3-E11
Continuity mistake: In the bar, before Bart stomps on Mr. Burns' foot, his back is toward Smithers, but in the next shot, his back is turned toward everyone else. (00:19:15)
Homer Alone - S3-E15
Audio problem: When Lisa screams "Mom" on the way to school, she just leaves her mouth open and doesn't close it. It should sound like "Maaaaaa" if she keeps her mouth open. (00:03:20)
Continuity mistake: During the Itchy and Scratchy cartoon, things change about the positions of the cats, the clothing, etc. (00:07:55)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Audio problem: In the opening when Marge is warning us about how bad this years episode is, her mouth barely matches what she says.
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: When the last wish is granted by Homer, when the index finger moves, you can see the refrigerator to the right of the hand. Homer was standing with his right arm facing the fridge. (00:06:55)
Continuity mistake: When we first see the papers on top of Skinner, the jar of preserves is not visible anywhere. It is beside him in the next shot. (00:19:15)
Continuity mistake: Bart's bruised eye has healed by the time he reaches Skinner's office to lick envelopes. (00:02:40)
Audio problem: When Bart says "Krusty Hotline" his mouth doesn't move.
Continuity mistake: When Mr. Burns is notified about the Leftorium, the can and can opener have disappeared from Smithers' hands. (00:19:45)
Mr. Lisa Goes to Washington - S3-E2
Audio problem: When the lady is asking questions to Homer before Lisa wins the trip to Washington, what she says doesn't match her mouth. (00:07:20)
Continuity mistake: When Mr. Burns notices that Homer is wearing a pink shirt to work, when he pauses and zooms in the video, Homer is looking almost directly at the camera. In the next shot, though, he is looking more to his left. The people surrounding him also change. (00:03:05)
Continuity mistake: After Marge says she will marry Homer, at the beginning of the scene, the car is facing the power plant, and at the end, it is facing away from the plant. (00:07:45)
Continuity mistake: When Skinner gets trapped under the papers, the jars on the shelves change color between shots. (00:19:15)
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