The Last Temptation of Homer - S5-E9
Continuity mistake: Mr Smithers throws a notepad at the person saying that a real emergency exit is required, but when we see it on the floor it is now a book with a green cover.
The Last Temptation of Homer - S5-E9
Continuity mistake: When Homer and Barney are at Moe's, Barney drinks his beer and then his cup his empty, and Homer's is full. Then later, Homer's cup is empty and Barney's cup has magically refilled itself.
The Last Temptation of Homer - S5-E9
Continuity mistake: When Bart has had his throat sprayed, we see him standing up just before the shot changes to the power plant. The shine on his shoes instantly disappears and reappears several times.
The Last Temptation of Homer - S5-E9
Revealing mistake: Just before Homer drives backwards into the trout hatchery, when Homer turns the key it is obvious that it is repeated footage of Marge turning the key because the hand is a lot thinner.
The Last Temptation of Homer - S5-E9
Continuity mistake: In this episode when Homer comes home from work, The Front door opens on the wrong side than it usually does.
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.
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