Other mistake: After Ugolin (one of the French men looking after Bart) has taken Bart to the Chateau Maison, Cesar and Ugolin speak. (they aren't visible in this shot) Ugolin says "Escape is impossible", but the subtitles say that Cesar says it and vice versa for when Cesar actually speaks.
Continuity mistake: When Bart is standing under the shelter of "Bon Dormir", the pavement on which he is standing is grey. But when he is talking to a policeman, the pavement is now a more darker grey.
Continuity mistake: When Cesar and Ugolin are eating their lunch, Ugolin's dish only has some potatoes in it. A few seconds later, the dish now has two sausages added to it.
Continuity mistake: When we see Lisa eating her dinner, she has a glass of water by her plate. But when we see a full view of the table, the glass has gone.
Continuity mistake: When Adil is speaking to Lisa at dinner at the Simpsons' house, Lisa has a knife and spoon by the side of her dish. In the next shot, both items have disappeared.
Continuity mistake: When Bart is sitting in the bike passenger seat on his way to the Chateau Maison in France, you can see the seat is coloured brown, but when he arrives at the Chateau, the seat is now blue.
Continuity mistake: When Homer is saying goodbye to Bart at the airport, there is a metal fence behind him that changes position between shots.
Continuity mistake: When Principal Skinner is speaking to Marge and Homer, Homer is lying on the settee and you can see there is nothing beside it, but in the next shot his beer can appears from nowhere.
Continuity mistake: When Principal Skinner rings the doorbell of the Simpsons' house, the handle on the door is pink. But when Marge is at the door, the handle is now light blue.
Continuity mistake: When Principal Skinner and his mother are with Bart and his friends after the toilet prank, you can see Bart has Milhouse, Richard and Lewis are beside him. In one shot, Bart is on his own and the other three boys have gone, and in the next shot, they are back again.
Continuity mistake: Bart throws in a microscope in the cupboard of his bedroom when his mother leaves, but in the next shot the microscope is nowhere to be seen.
Continuity mistake: When Marge leaves Bart's room, you can see there is a cap on the mirror on the drawers, but when we see a full view of the room a couple of shots later, the cap has gone.
Continuity mistake: When Bart discovers a pink box by his bed, you can see there is nothing hanging out from the drawer behind him, but in a close-up of Bart in the next shot, a sock suddenly appears.
Continuity mistake: When Homer falls down the stairs, the toy clown lands behind him and a close-up of Homer shows an orange plant pot by the stairs. But when we see a overhead view of Homer lying down, the plant pot is nowhere to be seen.
Continuity mistake: When Marge enters Bart's bedroom, on the wall you can see a picture of a space rocket and on it there is a red cloth. But when we see a close-up of Marge in the next shot, the cloth has gone.
Continuity mistake: When Agnes meets Bart and his friend, it cuts to Skinner, then to Bart's group and quickly back to Skinner. When it goes to Bart's group (before Skinner) Lewis is yellow like the rest, in all other scenes he is black.
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