Character mistake: In the scene where the imposter Krusty is robbing the Kwik-e-mart, Krusty's hair is blueish green just like the real Krusty, but when Homer is describing the robber he says the the crook has red hair. This is so you have more clues on who the real impostor is. The imposter Krusty is really Sideshow-Bob who has red hair. (00:04:20 - 00:05:05)
Continuity mistake: When Lisa is first seen petting the baby lamb, she is next to the fence with hay beside her. In the next shot, she's far away from the fence and there's no hay anywhere.
Continuity mistake: After Homer prepares the BBQ he holds out the steak and holds it back in as Lisa speaks. When the shot changes he's holding it out again, and the pile of meat behind him is smaller and has moved slightly under the window.
Continuity mistake: During dinner Marge has no glass to her right. When she says 'Why thank you Homie' a glass has appeared next to her.
The Two Mrs. Nahasapeemapetilons - S9-E7
Continuity mistake: When the garden is seen with Bart throwing Hymns into the fire Apu's mother starts to walk to him and she must walk through a curved green structure.In the next shot of Bart,the structure has gone.
Tree House of Horror X - S11-E4
Continuity mistake: The broken railing in front of Lucy Lawless returns to normal when the shot changes after the Collector lands on the computer.
Continuity mistake: In the very beginning of the episode, Bart places Santa's Little Helper's dish into the sink and begins playing with the faucets. Marge comes in, and the bowl magically disappears from the sink, and it's missing for the rest of the scene.
Treehouse of Horror XV - S16-E1
Continuity mistake: When Mr. Burns is lying on the table before the Simpsons enter his body, he is wearing briefs. However, when the Simpsons are inside him, he is wearing boxers. (00:15:55)
Lisa's Rival - S6-E2
Continuity mistake: During the diorama show, they cut to a shot of Milhouse, and his glasses are broken and taped together. But when he's shown a second time, they are back to normal.
Continuity mistake: Bart is putting pool balls while training with Lisa, the 2 balls remaining on the table change from white to a pink/red. (00:14:20)
Continuity mistake: At the start, when Nelson takes the cupcakes, you can see that after he has started eating them there are 4 left in the box. Just before he shuts the lid to drop it, you can just see the tops of 6 cupcakes.
Continuity mistake: Throughout the dinner scene, whenever Homer wants the syrup, it disappears and re-appears next to Lisa.
Continuity mistake: The man conducting the tour of the box factory tells the children that they will not be able to see a finished box, as the boxes are assembled in a separate location. However, moments earlier we saw a flat box going through a machine, and coming out finished.
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: On 'Whacking Day,' Homer is seen charging people to park on his lawn. But later when the snakes are lured to the Simpson house, there are no cars there.
Continuity mistake: When Skinner gets trapped under the papers, the jars on the shelves change color between shots. (00:19:15)
Papa's Got a Brand New Badge - S13-E22
Continuity mistake: When the family is sitting on the couch and trying to catch a breeze from the fan, Lisa's shoes disappear between shots.
Homer vs. Lisa and the 8th Commandment - S2-E13
Continuity mistake: When all the kids run out of the back door, Homer drops a blue box. In the next shot it is a yellow box with red lines on the edges.
One Fish, Two Fish, Blowfish, Blue Fish - S2-E11
Continuity mistake: When Homer is given his plate of sushi, there are four pieces on his plate. Moments later, before he begins to eat, he has six pieces on his plate. (00:03:35)
Continuity mistake: In the scene after Homer and Apu rescues the babies, Maggie is first seen wearing a bow. Then, when she's next seen, it's gone. When we see her one more time, the bow is back.
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