The Old Man and the C Student - S10-E20
Continuity mistake: When Homer offers Lenny some nuts a spring pops out and gets Lenny in the left eye, but the next time we see Lenny it is in his right eye.
Monty Can't Buy Me Love - S10-E21
Continuity mistake: Ralph Wiggum is seen riding the train inside the mall, but is seen some seconds later amongst the crowd of people still waiting to enter the store.
Monty Can't Buy Me Love - S10-E21
Continuity mistake: Bart is next to Homer when Marge says "Where did you get that champagne?", but in the next shot, a clown is in the spot Bart was.
Thirty Minutes Over Tokyo - S10-E23
Continuity mistake: Marge loses one of her shoes in the "lava" , but when they fall down, Marge wears two shoes again.
Thirty Minutes Over Tokyo - S10-E23
Continuity mistake: When Homer breaks into Flanders's house there's a shot of the refrigerator and nothing is on it. In a closer shot, there are tickets being held up by a magnet.
Thirty Minutes Over Tokyo - S10-E23
Continuity mistake: When Database is at The Java Server, he is using a teal computer, but when Snake comes in, he is using an orange computer, then he is using a teal computer again.
Thirty Minutes Over Tokyo - S10-E23
Continuity mistake: When Lisa gets the tickets on the "volcano", she is a few inches away from the end of the bridge. When it falls and Marge runs onto the bridge, she is now in the middle of it.
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