Diatribe of a Mad Housewife - S15-E10
Continuity mistake: When Homer goes to Ned to make sure Marge imagines nothing again, Homer puts his hand through the door, then he puts his hand through the mailbox, then Nelson puts his hand out to steal the doormat. Nelson puts his hand just over the mailbox, then in the next shot when it reveals Nelson is there, his hand is suddenly under the mailbox without enough time to move it.
My Big Fat Geek Wedding - S15-E17
Continuity mistake: When Milhouse is hit by the rock in class, the map behind him changes colors. This actually happens within the same shot between frames.
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