Continuity mistake: The photo Edna sends to Bart differs when he shows his parents. The angle is different from what it was previously, revealing part of a photo frame on the wall which wasn't there before, and the sheets/pillows on the bed are different, and her eyes are closed when they look at it, but when Bart looked at it, they were open. (00:09:05 - 00:18:25)
Continuity mistake: When Edna waits in The Gilded Truffle, she checks her watch. In the next shot the watch has gone. (00:15:40)
Continuity mistake: When Edna sits at her desk saying "Oh, Woodrow. How could you stand me up?" the clock reads 3 O'clock. Seconds later, as Bart talks with Edna, the clock reads 4 O'clock. (00:16:45)
Continuity mistake: When Homer stands on the nail in the back yard, you can see behind him, that on the wall next to the back door there are two purple lights. Seconds later when Lisa shows him the new doghouse, the lights have gone. (00:17:25)
Continuity mistake: The notepad Homer draws the blueprint on changes from having content on it to being blank so he can draw on it.
Continuity mistake: The waiter at The Gilded Truffle who takes the chair away turns into a completely different waiter when the angle changes.
Continuity mistake: When Burns first shows the team the nerve tonic, the "&" on the 'Brain & Nerve Tonic' label is facing backward, but as Burns pulls it away it faces forward. Then as he moves it around as he's talking, the "&" switches between facing forward and facing backward again. This happens all in the one shot.
Continuity mistake: At the beginning of the episode, the school is a light purple colour, yet near the middle of the episode, the school turns yellow. (00:00:50 - 00:12:20)
Continuity mistake: In the scene at the beginning when the Simpsons are eating dinner, Marge and Homer's plates are empty, then they have food on them, then they are empty again. (00:03:40 - 00:04:30)
Continuity mistake: Near the beginning when the family is eating dinner, the glasses on the table keep refilling/emptying themselves, even though nobody takes a drink. (00:03:40 - 00:04:30)
Continuity mistake: At Li'l Ludwig's Music School, when Lisa plays her sax she has a rope around her neck, attached to the sax. Then when Lisa yells "You're wrong. You're wrong." The rope has vanished, but is around her neck in the following closeup. (00:05:05)
Continuity mistake: When Lou and Eddie (the police officers) arrive at the house to take Bart for a drive, their badges are white. When they are driving in the cop car, their badges are yellow. (00:05:30)
Continuity mistake: When Marge is talking to Lisa in her room about Band Practice, Lisa is reading comics, eating potato chips, and there is a drink by her as well. The drink container starts out green with a light purple label, then goes to being grey with a red label then as Marge tells her story, the colours of the drink change again twice. (00:10:00 - 00:10:55)
Continuity mistake: When Millhouse spits the paper at Kearney in the cafeteria, Jimbo is seen wearing a black shirt and Dolph is wearing a green shirt yet, when Millhouse is dragged out the cafeteria by Bart, they are both wearing blue shirts. (00:16:10)
Continuity mistake: In the cafeteria scene where Milhouse shoots a spitball at Kearney, in the shot when Milhouse says, "We have order, but at what price," Milhouse, Lewis and the bullies all change positions at their tables. You may have to watch this part several times to understand it. (00:16:45)
Continuity mistake: When Lisa is clapping erasers because she sassed Miss Hoover a second time, the room's clock is visible near the top of the ceiling. However, the clock wasn't there several shots earlier. (00:17:00)
Continuity mistake: In the scene where Miss Hoover asks Lisa what the ninteenth-century figure nicknamed 'Old Hickory' is, there is no text on the covers of the book she's holding. In the next two shots, however, 'Teacher's Edition' can be seen on The Front cover and the spine of the book. (00:17:00)
Continuity mistake: When Bart and Skinner are checking lockers, you can see in a wide shot that Bart needs to check seven lockers before he gets to Lisa's locker. However, Bart checks eight lockers and then opens Lisa's locker. (00:19:35)
Continuity mistake: If you watch carefully, the same footage of Bart and Skinner slamming locker doors is used. The easiest to spot is the shots of the camera at Bart from inside a locker, where a kid's scrapbook is on the left side. (00:19:45)
Continuity mistake: After Bart finds out that Lisa took the teacher's books, his Hall Monitor sash disappears and reappears for several shots. (00:20:05)
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