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.
Catch 'em If You Can - S15-E18
Continuity mistake: When Lisa says "It's called Wicca, and it's empowering" to Bart, her necklace is red instead of white.
Continuity mistake: When Duffman says "Duffman can never die, only the actors who play him", his red briefs are missing.
Sweets and Sour Marge - S13-E8
Continuity mistake: When Homer dumps the sugar into the ocean, the hand pressing the "dump cargo" button has a yellow shirt cuff. Homer is wearing a white short-sleeved shirt.
Sweets and Sour Marge - S13-E8
Continuity mistake: At court, when Garth Motherloving tells Judge Snyder "I'll kill you while you sleep", the blue haired lawyer's suit is green. His suit is grey in all other shots of him.
Continuity mistake: When Lisa comes and starts giving out apples, Moe is seen taking one of them, but some seconds later, he is seen under Homer's coat with a piece of Homer on a fork.
I'm Goin' to Praiseland - S12-E19
Continuity mistake: Helen Lovejoy suddenly has lipstick on when she gives Homer the ice cream. She doesn't have lipstick on in previous shots.
Continuity mistake: Lenny's mug refills itself when he tells Homer, "You want to help me?"
Continuity mistake: After Amy Tan shames Lisa, there is a shot of Lisa wearing a red necklace, her necklace becomes her usual white one, seconds later.
Kill the Alligator and Run - S11-E19
Continuity mistake: The 2 guitarists in Kid Rock's band swap hair color when Kid Rock first arrives on stage.
Continuity mistake: After Lisa wakes from her daydream, there's a boy behind her with blonde hair and a light purple shirt. After Ms. Hoover says "Then come back and finish the test", the boy's hair turns blue, and after it zooms in on Lisa's face, there's a boy with brown hair and a blue shirt behind her.
Missionary: Impossible - S11-E15
Continuity mistake: When Q'Toktok says "I told you we should have asked them", his hair is black instead of grey.
Continuity mistake: During the revival meeting, Ruth Powers has a bandana on in some shots, and doesn't have it in other shots.
Continuity mistake: Homer was wearing a knit cap when the bucket fell on his head, but didn't have it when the bucket was removed. And since he was wearing a knit cap when the bucket fell on his head, his hair shouldn't have been ripped off when the bucket was removed.
Continuity mistake: In the dinner scene at the end, Maggie is not wearing a bow, but when everyone tells Moe "Merry Christmas", she has her bow on.
Monty Can't Buy Me Love - S10-E21
Continuity mistake: Ralph Wiggum is seen riding the train inside the mall, but is seen some seconds later amongst the crowd of people still waiting to enter the store.
Monty Can't Buy Me Love - S10-E21
Continuity mistake: Bart is next to Homer when Marge says "Where did you get that champagne?", but in the next shot, a clown is in the spot Bart was.
Continuity mistake: Milhouse's hair is blonde in the school bus' mirror reflection.
Wild Barts Can't Be Broken - S10-E11
Continuity mistake: During the song, Reverend Lovejoy at first has a blue sweater over his usual pink shirt, then he has a blue shirt, then he has his pink shirt without a sweater.
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