Other mistake: When Bart is looking at Doctor Hibbert on the rowing machine, all the pictures on the walls behind him don't have faces on them.
Other mistake: When Bart is looking at space through his telescope, Earth can be seen through the telescope despite him being on Earth.
Tree House of Horror V - S6-E6
Other mistake: At the end, just before the Simpsons are turned inside-out, Bart is not only wearing his red shirt in bed, but sitting in a position that's not even possible in any position he was sleeping in. (00:20:35)
Tree House of Horror V - S6-E6
Other mistake: At the speed that Homer was chasing the family outside the lodge at the end of "The Shinning", he would have caught them up with ease.
Lisa on Ice - S6-E8
Other mistake: As Homer is signing Bart's academic alert form just before Marge starts talking to Lisa you can read the writing below the "Academic Alert" title, but the writing is completely incoherent including sentences such as "IMAEGTSIAD TSNHOJI NAMREB". Update; some of the "words" or parts of them are people's names, such as David, Stuart and Allison, but backwards. (00:03:50)
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
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.
Homer vs. Patty and Selma - S6-E17
Other mistake: When Homer burns a dollar bill, he turns his cigar around 180 degrees. If you look carefully, the smoke soon comes from the different side than it did earlier.
Who Shot Mr. Burns? (1) - S6-E25
Other mistake: In the very first scene, when Principal Skinner is opening the door to the school, there is no name on the school. (00:00:25)
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