Continuity mistake: All of the cigarettes that Moe has smoked disappear from the floor after Marge has spoken to him.
Continuity mistake: The "Stinger" only appears on the boxing gloves after Moe passes them to Homer. It is not there in all preceding shots.
Continuity mistake: The extension cord Jimbo is using for a belt disappears after they run around the corner of the corridor whilst chasing Bart.
Character mistake: Moe says that no women have been to his bar since 1979, yet in "Flaming Moe's" he hires a female waitress. Also, in the episodes "New Kid on the Block" and "Homer the Vigilante," Ruth Powers appears in the bar, and orders a beer in the former episode while Marge visited the bar in "Bart Gets Hit By a Car."
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