Continuity mistake: In the Itchy and Scratchy cartoon 'A Briss Before Dying', Itchy throws Scratchy's eyes out of the window, but after putting the rest of his body through the grinder the eyes are back.
Continuity mistake: The coathanger is too small to reach the medicine cabinet.
Continuity mistake: After Lisa unlocks the bathroom door with the coat hanger, the puddle of "baby blood" (Pepto-Bismol) at her feet disappears in the next shot.
'Tis the Fifteenth Season - S15-E7
Continuity mistake: After the family discover the astrolobe, the gift tag disappears from its wrapping. (00:07:25)
'Tis the Fifteenth Season - S15-E7
Continuity mistake: Homer looks out of his bedroom window at the skating rink he has built in his back garden. Homer and Marge's bedroom is usually at The Front of the house. (00:13:50)
'Tis the Fifteenth Season - S15-E7
Continuity mistake: When we first see the skating rink, it has Jasper, Disco Stu, Otto, Apu and Manjua, Sideshow Mel, a couple of children and two couples skating on it. In the next shot we see the Comic Store Guy split his trousers - he wasn't there before. (00:14:00)
'Tis the Fifteenth Season - S15-E7
Continuity mistake: After Homer decides that he doesn't want to sleep with Marge, he goes downstairs and watches TV. He moves to a different side of the sofa between shots.
Marge vs. Singles, Seniors, Childless Couples and Teens, and Gays - S15-E8
Continuity mistake: In the bathroom, Lisa can't stand the Roofi CD being played so she pulls the plug out of the plug socket to stop the radio playing. In the next shot, the plug has disappeared from the radio. (00:02:15)
Marge vs. Singles, Seniors, Childless Couples and Teens, and Gays - S15-E8
Continuity mistake: When Homer sneaks into Maggie's room to smash the CD player, there is a yellow box with musical notes painted on it. The notes are different in each shot. (00:02:30)
Marge vs. Singles, Seniors, Childless Couples and Teens, and Gays - S15-E8
Continuity mistake: When Bart asks Ms. Krabapple for detention, the piece of paper on his desk disappears. (00:03:05)
Marge vs. Singles, Seniors, Childless Couples and Teens, and Gays - S15-E8
Factual error: The symbols on the heads of the Teletubbies on stage are wrong. They did use the word 'teletubbies', so they're meant to be the same as the real world. (00:05:40)
Marge vs. Singles, Seniors, Childless Couples and Teens, and Gays - S15-E8
Continuity mistake: When we see The Town at the meeting, Smithers is sitting in front of Selma. In the next shot, though, Smithers is suddenly on stage holding the 'SSCCATAGAPP' sign and Lenny is sitting in front of Selma. (00:08:50)
Marge vs. Singles, Seniors, Childless Couples and Teens, and Gays - S15-E8
Audio problem: When Marge says "Save our families" at the beginning of her petition, her mouth doesn't move. (00:14:40)
Marge vs. Singles, Seniors, Childless Couples and Teens, and Gays - S15-E8
Continuity mistake: When Roofi is singing at the concert, the Roofi head on the curtain is facing the left. Later, it is facing the right.
I, (Annoyed Grunt)-bot - S15-E9
Continuity mistake: When the boys get to the top of the hill the light on The Front of the bike has shrunk dramatically since the first time we saw it at Bart's driveway. (00:00:55 - 00:02:15)
I, (Annoyed Grunt)-bot - S15-E9
Other mistake: When the bullies are riding around Bart and Milhouse there are problems with their water bottles. In some frames they vanish, in some others the bullies' far legs are drawn in front of the bottle. (00:01:40)
I, (Annoyed Grunt)-bot - S15-E9
Continuity mistake: The cats' graves change position. When Snowball II is buried, they're away from the the tree and near the fence. When Snowball III is buried, the graves are away from the fence and Snowball is among the roots of the tree. (00:04:10 - 00:09:25)
I, (Annoyed Grunt)-bot - S15-E9
Audio problem: After watching 'Robot Rumble', Bart tries to dissuade Homer from building a robot by listing his other "talents". The voice and Bart's mouth movements don't at all match. (00:06:20)
I, (Annoyed Grunt)-bot - S15-E9
Continuity mistake: The remote control for the robot in Bart's bedroom appears from nowhere. (00:08:30)
I, (Annoyed Grunt)-bot - S15-E9
Continuity mistake: When Snowball III drowns, the number of weeds and rocks in the fishtank changes between shots. (00:09:15)
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