Factual error: In the episode with Donny, when Bart sees principle Skinner go into the school to meet with the rat, the door is backwards. The bars that are pushed in to open a door are always on the inside.
I Don't Wanna Know Why the Caged Bird Sings - S19-E4
Factual error: During Dwight's attempted bank robbery, he gets hit with a couple of dye packs, then appears a few shots later totally clean. The dye packs banks use are notoriously difficult to wash off once they're triggered; even if Dwight did manage to clean himself up, there would still be remnants of dye on him.
Treehouse of Horror XVIII - S19-E5
Factual error: In the Mr. And Mrs. Simpson segment Homer is firing a minigun at Marge and three of the barrels are seen spitting fire. A minigun however only uses one barrel at a time to fire.
Factual error: The cover of "World's Finest Comics" #94 is shown on the wall at the comic book store. It features Superman with a sign that says, "Gotham or Bust," and Batman gulping. The actual cover of "World's Finest Comics" #94 shows Superman being sprayed with liquid kryptonite while Batman and Robin catch sight of him.
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