Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: Homer leaves the Simpson house to go to the farm house in the station wagon, and when he parks in front of the farm house he's driving his pink car.
Grandpa vs. Sexual Inadequacy - S6-E10
Other mistake: Look closely when the clerk scans the books. Ueberroth is a brown book, and Al Gore's book is green. The book that sends the message to Al Gore is the Ueberroth book.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: While hiding the fact they are looking at sex books from Bart and Lisa, Homer picks a green book. When we see what he's reading, it's a grey "Maple Thorpe" book.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: On the book on conspiracy theories that Bart buys, there's a picture on the back of the author. The picture changes several times throughout the episode.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: When Homer and Grandpa are trying to sell the tonic at the mall, when Homer is trying to sell tonic to the Critical Man, all the tonic bottles on the table are actually empty. In the next shot, they're full.
Grandpa vs. Sexual Inadequacy - S6-E10
Revealing mistake: When Doctor Hibbert drinks the bottle of tonic, the amount of tonic never actually changes even though it should have gone down a considerable bit.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: When Lisa jumps off the tyre swing, her dress is fairly dirty. When Homer drives off, the bottom is dirtier than before.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: At the old Simpson home, Grandpa and Homer run into each other and collide on the porch. In this shot, the windows are dark, but in the next, all the windows are instantly lit up by the fire, which would not be possible in the given time.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: When the janitor shows Marge and Homer into the Utility Room, the only equipment in there is a couple of mops. Later, the janitor comes back and pulls out a big vacuum cleaner that we never saw previously.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: At the retirement home, Grandpa pulls out some medicine from his cupboard. In the next shot, another medicine bottle that wasn't in the previous shots appears.
Grandpa vs. Sexual Inadequacy - S6-E10
Other mistake: When Grandpa throws the bottle to Homer, look at the bottle - at the angle it was thrown at, it should have missed Homer completely.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: In one scene in Bart's treehouse, Wendell's skin is yellow, rather than its usual white color.
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