Kamp Krusty - S4-E1
Continuity mistake: In the shot where Kearney is reading the letter sent by Marge whilst he is eating the jellybean cookies, there is a quick shot of Kearney's hand going to his mouth with the cookie to eat it. Note that there is nothing on his wrist. In the next shot where it is revealed that Kearney is reading the letter rather than Bart or Lisa, he has spiky, black collars on his wrist. (00:14:25)
Kamp Krusty - S4-E1
Continuity mistake: When the kids rip Krusty's shirt to see if it is the real him he buttons his shirt up then in the next shot his shirt is still torn.
Marge vs. the Monorail - S4-E12
Continuity mistake: Just after the Monorail has been seized by the "anchor" getting caught on the donut sign, the long stripe at the top of the side of the Monorail changes from being red to purple.
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: When there is a queue to see the movie, it stretches right back to the Simpson house, and we see Bart looking through the window. However, when Homer talks to Bart whilst sitting on the window, the queue has disappeared. (00:16:45)
Brother From the Same Planet - S4-E14
Continuity mistake: When Homer is showing Pepi the garage, it is purple. In the main titles and in every other episode, it is an orangey colour.
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: When Bart and Lisa are watching TV, Bart says something like 'I've got my ticket' and pats a ticket that is inside his shirt pocket (which there is none). But later in that same scene when they are still watching TV, both the pocket and the ticket have gone. (00:12:20 - 00:14:15)
Continuity mistake: The old men beside Grandpa when Homer plows the retirement home switch places, depending on whether it's a long or a close shot. (00:10:00)
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: When Maggie is driving Homer's car, a man is behind him. He is seen waving his left arm but when the shot changes to Maggie he is waving his right arm. (00:14:50)
Kamp Krusty - S4-E1
Continuity mistake: Homer points out to Marge that now that the kids are away, he has grown two more hairs, making it four on the top of his head. Later when they are reading Lisa's letter, he has two hairs again. But later still when they are watching TV, he has four again. Two of them fall out when he see that Bart is the ringleader, leaving him with his original two. (00:15:35)
Homer's Triple Bypass - S4-E11
Continuity mistake: When, Lisa bangs on the window of the operating theatre, there is a person sitting behind her who is asleep. However, when she shouts to Dr Nick, the people behind her are completely different. Then she sits down again, and the sleeping person is behind her. (00:20:00)
Homer's Triple Bypass - S4-E11
Continuity mistake: When Krusty takes off his shirt, the superfluous third nipple and cattle skull birthmark shown in "Kamp Krusty" are not on his chest. (00:14:50)
Continuity mistake: When the family is at dinner, there is a top down view which zooms in on them from above. In this shot there is no newspaper on the chest (to Homer's left). In the next shot of Homer a newspaper has appeared. (00:07:50)
A Streetcar Named Marge - S4-E2
Continuity mistake: As Marge is about to leave for her first audition, she walks across the room and she is wearing white slip-on shoes. Then her and Homer talk for a bit, then when it cuts to a wider shot after they talk, she now has her red flat-heels on. (00:03:30 - 00:04:00)
Homer's Triple Bypass - S4-E11
Continuity mistake: When Dr Nick visits Homer for the first time, he leaves the door open. But later before the reporters come to the door, the door is closed. (00:13:40)
Continuity mistake: In one scene Bart is swinging on a washing line then we see Homer talk to Marge and he says "He'll soon get tired of it" then it goes back to Bart and he is facing the opposite direction and he is swinging on two washing lines. (00:04:22)
Continuity mistake: When the police talk about the bumper cars getting stolen by Jimbo Jones, you see him and his 2 friends riding off in bumper cars. From the side angle, there are 2 blue and 1 red car, but from the rear view, there is 1 blue, l red, and 1 green car.
Continuity mistake: In this episode Hans Moleman's skin colour is yellow. However in "Duffless " his skin colour is brown and in other episodes it is a dark yellow-green colour.
Continuity mistake: When it is announced that Amber is the winner of the contest, and Lisa is runner-up, the stands in the back of the stage are empty. However, when Lisa walks off, all the contestants are back on the stands. (00:15:00)
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