Lisa's Date with Density - S8-E7
Continuity mistake: When Jimbo and Co are 'coleslawing' Skinners house, he wakes up when coleslaw hits his window. When the shot moves to outside, there isn't any coleslaw on any of the windows.
Lisa's Date with Density - S8-E7
Continuity mistake: Whilst Chalmers notices the Honda symbol missing from his car, there are four holes in the corners of the leftover symbol. There is a screw in one of the holes indicating the symbol would have holes in its corners to hold it down. Later when Skinner finds the Honda symbol in Nelson's locker you can see there are no holes in the symbol.
Lisa's Date with Density - S8-E7
Continuity mistake: The hole in Nelson's locker increases in size.
Lisa's Date with Density - S8-E7
Continuity mistake: As Homer is walking out of the Kwik-E-Mart, after Apu refuses him the donut, he has nothing in his hands. In the next shot from outside the store we see him carrying a small paper bag.
Lisa's Date with Density - S8-E7
Continuity mistake: In Lisa's music class, there is a wide shot of the entire class. Üter sits next to Milhouse Van Houten in this shot. However, in the next shot, it's Ralph Wiggum. In a following shot of the entire class, it's Üter again. They both replace each other in the same seat.
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