Continuity mistake: When the Simpsons are on the raft going down the river in Africa they come to a fork and have a choice of going to the left, towards a sunny and nicer area or to the right where it is rainy and gloomy. As Homer tries to steer the raft to the left as he pulls his left arm out of the water and it is covered in leeches but in the next shot of him when we see the arm again the leeches are gone. No way to remove leeches that fast.
Continuity mistake: Milhouse is seen in the school bus during the Lisa segment, even though he was with Bart at the time.
Audio problem: When Homer and Marge are driving to the hospital, Marge says "Let's just say that Bart did it". But her mouth says something else when she says "Bart".
Continuity mistake: The bikes disappear after Bart and Milhouse arrive at the cave.
Audio problem: When Lisa says "Oh no, somebody took my bike", she is mouthing something else.
Continuity mistake: During Lisa's and Bart's segment, time seems to run really quickly. When Lisa leaves West Springfield Elementary it is 11:00 and when she gets to Moe's it is noon, then a few minutes later it is 2:50. When Bart and Milhouse find Fat Tony it is noon, then when Bart gets hit by the truck, it is almost 3:00.
Audio problem: When Bart is going through his keys, he says "Your house", and his mouth doesn't match what he is saying.
Audio problem: When Marge says "We've gotta get to the hospital, Homer" her mouth doesn't move.
I'm Goin' to Praiseland - S12-E19
Continuity mistake: When Flanders' girlfriend pulls up at the end, from one angle she's wearing a pink shirt with what looks like a white vest over top. But from another angle her shirt is all white.
I'm Goin' to Praiseland - S12-E19
Revealing mistake: In one of the scenes (I think it is when they first discover the alleged 'miracle') you can see people in the background walking out of the park. But they are not walking out of the park at all, they are walking into the wall.
I'm Goin' to Praiseland - S12-E19
Continuity mistake: When we first see the "Whack-a-Satan" game at Praiseland, there are three rows of holes the little devil dolls emerge from. After Nelson turns towards Flanders, the game is seen behind him, now with only two rows of holes.
I'm Goin' to Praiseland - S12-E19
Continuity mistake: Helen Lovejoy suddenly has lipstick on when she gives Homer the ice cream. She doesn't have lipstick on in previous shots.
I'm Goin' to Praiseland - S12-E19
Continuity mistake: When the citizens are leaving the park in disappointment, Manjula walks past the screen and has yellow skin, light brown hair and is wearing a blue dress. In the next shot she is back to her normal skin color and is wearing her usual pink dress.
Children of a Lesser Clod - S12-E20
Continuity mistake: When Ned Flanders is asking Homer whether or not he is running a daycare centre, there is a radio behind him. The aerial on it changes position in subsequent shots.
Children of a Lesser Clod - S12-E20
Continuity mistake: When Bart and Lisa are trying to win back Homer's affection they are sitting on the top of the stairs and push the skateboard with the present on it down the stairs. In the next shot it rolls into Homer's legs yet it would be impossible as he was standing in the main living room which is to the right of the stairs. Pushing the present down the stairs it would go straight into The Front door.
Children of a Lesser Clod - S12-E20
Audio problem: When the moon says, "You go, Homer,'" the words dont match what the moon says.
Continuity mistake: When Lisa comes and starts giving out apples, Moe is seen taking one of them, but some seconds later, he is seen under Homer's coat with a piece of Homer on a fork.
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