Continuity mistake: When at the dinner table, Marge tells Homer and Bart that they can go to see Truck-o-saurus. In one shot Bart has his hands on the dinner table, but in the next shot his arms are by his side. (00:04:20)
Continuity mistake: When Homer is driving to Truckasaurus there is a wide shot of the road with two cars on it, but in the next side shot of the road, the road has loads more cars on it. (00:06:30)
Continuity mistake: When Bart attempts to skate over Homer's car, his three mates are watching. Between shots they change from standing on the driveway to standing on the pavement. (00:10:55)
Continuity mistake: When Bart skates towards Homer's car, attempting to skate over it, the distance between the car and Bart changes between shots. (00:11:00)
Continuity mistake: When Bart falls off his skateboard after attempting to skate over Homer's car, as the shot zooms out you can see his board is nowhere to be seen. (00:11:05)
Continuity mistake: When Homer falls down the gorge, the medics lift him up on a bungee and to the top of the cliff. Between shots the edge of the cliff changes from green grass to brown rock. (00:20:05)
Continuity mistake: The guardrail at the Springfield Gorge keeps changing between shots when Bart is about to make his big jump; in the first wide shot there is a short rail opposite the "stone ramp." Then in the Bart-POV shot from atop the ramp, the rail is much longer, and the parking lot and picnic area behind it have disappeared. And when Homer is actually jumping the gorge, the rail disappears altogether.
Itchy and Scratchy and Marge - S2-E9
Continuity mistake: In the beginning after Maggie hit Homer on the head, it shows him laying on the couch with Bart & Lisa on the left. The next shot it shows the TV, in the next shot it shows Homer on the left sitting and Bart & Lisa on the right. It's impossible for Homer to get up and switch sides with Bart And Lisa in less than 5 seconds.
Bart Gets Hit By a Car - S2-E10
Continuity mistake: In Moe's Tavern, in the close-up of Marge as she says "Homer, look at me" the glass window behind her has a different pattern than in all the other shots (2 rows of red tiles instead of 3).
Bart Gets Hit By a Car - S2-E10
Continuity mistake: During Marge's testimony, Lionel Hutz has a neat stack of papers in front of him the whole time. But in the close-up when Homer and Hutz both cover their eyes in sorrow, Hutz has several pieces of crumbled paper and a broken pencil in front of him. The was not nearly enough time for him to make such a mess since the last shot of him.
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)
One Fish, Two Fish, Blowfish, Blue Fish - S2-E11
Continuity mistake: Just after Homer says, "This fish is delish", one of the pieces of sushi keeps reappearing every time Homer eats it.
Continuity mistake: While Homer and Barney are smoking in the boys bathroom, when Principal Dondelinger walks in on them the "boys" sign on the exterior side of the door disappears in the closeup, but the sign reappears as Dondelinger exits. The door push plates on both sides of the swing door also vanish/reappear during the scene. (00:04:35)
Continuity mistake: When Homer shows up to the Bouvier home the night of the prom, Selma and Patty open the door and it opens outward, but in the following shots the door has been opened inward. (00:16:10)
Continuity mistake: When Homer stands outside the Bouvier house to pick up Marge, the home's front door has three diagonal windows from upper left to lower right. However, when Marge gets home later after the prom, the three windows are diagonal the opposite way from upper right to lower left. (00:16:10 - 00:21:00)
Continuity mistake: When Marge comes downstairs she's shocked to see Homer in her house, and when the doorbell rings we see Mrs. Bouvier wearing an orange belt with a red buckle, but then the colors switch to an orange buckle and the belt center is red. (00:16:45)
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.
Homer vs. Lisa and the 8th Commandment - S2-E13
Continuity mistake: When Homer and Marge are discussing about unhooking cable, the time on the clock on Homer's bedside table changes between shots.
Homer vs. Lisa and the 8th Commandment - S2-E13
Continuity mistake: When all the kids are watching TV, we see Wendell, Martin and Milhouse. Absurdly they appear to be right below the ceiling in this shot - when in every other shot they are sitting on the sofa or standing.
Homer vs. Lisa and the 8th Commandment - S2-E13
Continuity mistake: In every other episode, the bar at Moe's Tavern is a regular L-shape. In this episode, it has somehow gained two additional angles on the long side, making it a Z-shape (plus the angle to the short end, which is not shown).
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