Audio problem: After the donut delivery guy asks Carl why all the donuts are piling up, Carl responds with Lenny's voice.
Continuity mistake: When Mr. Burns and Smithers go to leave, the beak-like knocker has disappeared from the Simpsons' front door.
Continuity mistake: When Marge goes in front of her painting after it is uncovered she stops with her head in front of Mr. Burns' genital area. In the next shot, her head is shown in front of Mr. Burns' hand.
Continuity mistake: In the scene where Bart and Homer find Marge's paintings in the attic, there is a shot of the paintings all stacked up and you can see that the edges of all of them are the same shade of pink as The Front painting. However, when the boys flick through the paintings, the backgrounds and edges are a variety of colours, including pale blue.
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