Continuity mistake: When everyone discovers the monster is a boar, there is a Chippos packet on its tusk. But when the boar is shown licking the slime, the Chippos packet has disappeared without ever being touched.
Continuity mistake: Just before Homer meets Bill Gates, and during the meeting, the pens and pencils in the butter alternate between shots.
Continuity mistake: Throughout the trial, the twins keep switching places between shots. Note - one twin has a bow in her hair and the other twin doesn't.
Continuity mistake: At the start of the trial, Nelson is seen sitting in front of Martin and one of the twins. But in the shot after Lisa says Milhouse is the only witness, there is no sign of Nelson.
Continuity mistake: When Bart says "No talking in the cage", both him and Milhouse are right next to each other. But two shots after Lisa says Milhouse is the only witness, Milhouse is a good 10 foot away from Bart without ever having moved the cage he was in.
Continuity mistake: Everyone discovers Milhouse ate the food so they walk up to him. The shot cuts and they are further away than they were in the previous shot. (00:13:50)
Continuity mistake: When everyone discovers Milhouse has eaten the food, Milhouse then says "You wouldn't dare hurt me. You forget, I have the glasses." After this line, Nelson takes off Milhouse's glasses. At this point, a rip is visible on the left side of Nelson's shirt. Milhouse then walks backwards and Nelson is seen re-entering the shot from behind Milhouse. Only now, the rip in Nelson's shirt is now suddenly on the right side of his shirt.
Continuity mistake: When everyone sees Milhouse after they find out the food has disappeared, Milhouse's arms are by his head. In the next shot, one of his arms is suddenly on his belly.
Continuity mistake: When everyone notices no food in the cooler, the cooler lid is raised, then it is down, then it is suddenly raised.
Continuity mistake: When everyone takes food out of the cooler, Bart has a bag of OREOs which magically becomes a bag of different cookies.
Continuity mistake: When Bart comes out of the water with the cooler, he doesn't have a shirt on. At no point does he put his shirt back on, it reappears when Lisa takes the food off everyone. (00:12:45)
Continuity mistake: As soon as the fish bites Bart's butt and causes a rip in his shorts, the rip disappears almost instantly. (00:12:40)
Continuity mistake: There are 10 kids stuck on the island. But after the kids have washed up on the island and it cuts to the wide shot and it shows them on the beach, there are only 6 kids. (00:07:20)
Continuity mistake: In the shot before Nelson says "Hey, Simpson. Race you", Ralph is seen sitting on the row in front of Martin. But when Ralph says "Go, banana" he is suddenly sitting two rows in front of Martin. (00:05:25)
Continuity mistake: When Bart and Nelson first roll the orange and apple, we can see there is no-one sitting in front of Nelson. But a couple of shots earlier, we saw Milhouse sitting in front of Nelson.
Continuity mistake: On the bus, Otto rewinds his tape and there is a shot of the kids. In this shot, we can see the kids and nothing else. A couple of shots later, a food box has appeared out of nowhere.
Revealing mistake: At the U.N. meeting, Bart places his glasses down on the table to read out the Libyan exports. As he puts the glasses down, you can see they are unrealistically floating in the air.
Continuity mistake: At the Model U.N. meeting, Bart finishes speaking and puts down his papers on the right hand side of his desk. Two shots later, the papers are on the middle of the desk. (00:03:10)
Continuity mistake: The pens and pencils in the good butter change between shots when Homer begins his website. (00:04:55)
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