The Fat and the Furriest - S15-E5
Continuity mistake: When Homer realises that the bear was only agressive because of the electrical tag, they share a bear hug. The back of Homer's shirt is torn to tatters, and his back is bleeding. Immediately thereafter, the back of Homer's shirt is completely intact.
The Fat and the Furriest - S15-E5
Continuity mistake: When Homer and the bear face each other in the cave, he's barefoot, but when he removes the electric prod from the bear's ear, he inexplicably wears shoes, and then he's barefoot again when he puts it on his ear.






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