Continuity mistake: When Cameron is being booed, Lenny's five o'clock shadow is missing, it returns in the next shot.
Continuity mistake: After Lisa's final performance, Bernice Hibbert (sitting next to Dr. Hibbert) has light brown hair instead of black/dark brown.
Continuity mistake: After Lisa's last performance, Carl and Lenny are seen with grey hair.
Continuity mistake: After Lisa finishes singing her song during the audition, there are several coloring mistakes in the shot of the audience clapping. Agnes Skinner's face is ghost white, but her hands are yellow. Mayor Quimby's suit is lavender instead of violet/blue, and his hair is light brown instead of medium brown. Helen Lovejoy has blonde hair instead of grey, and is wearing a purple shirt instead of pink sweater over blue shirt, and Reverend Lovejoy has a green shirt instead of pink, and has brown hair instead of black.
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