Continuity mistake: When Homer spots the penny, he moves his foot and there is nothing on the floor. When the shot changes, the penny has appeared. (00:06:50)
Continuity mistake: Homer asks the family how church was. When he does, Marge grabs her nose. Camera cuts and she is no longer holding her nose. (00:07:30)
Continuity mistake: When talking to the family about no longer attending church, Bart moves behind Homer in an instant cut. (00:07:45)
Continuity mistake: Homer says he's had the best day by not going to church. When he does, Marge has Maggie in her arms. When the camera changes to behind, you can see she no longer is holding Maggie. (00:07:45)
Continuity mistake: Homer says he wasn't sure if he was doing the right thing in regards to giving up doing to church. When he does he puts his fork down across his knife. When the camera cuts the two utensils are no longer crossed. (00:10:40)
Continuity mistake: While Homer is sleeping, the fire burns off the hair on his head. But when he wakes up and jumps up off the couch, his hair is back again. (00:17:00)
Revealing mistake: When Lisa says "Truly this was an act of God", her ear disappears and reappears several times. (00:19:30)
Continuity mistake: Throughout the episode, God has 5 fingers. At the end, Homer asks what the meaning of life is, and God has only 4 fingers now. (00:21:20)
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