The Great Louse Detective - S14-E6
Continuity mistake: When we see the picture collage of Moe with the pickle jar the bottom picture is of them in front of the pyramids. When it pans around the pictures though, it turns into a picture of them on the beach.
The Great Louse Detective - S14-E6
Continuity mistake: When Bob pins Bart onto the wall in the final scene, his pajama top is stretched up so you can see his stomach. It then zooms in slightly when he looks at his watch and his stomach is covered.
The Great Louse Detective - S14-E6
Continuity mistake: Frank Grimes' gravestone appears to look different from "Homer's Enemy" and "Alone Again, Natura Diddly."
Special Edna (a.k.a. Love and Marking) - S14-E7
Deliberate mistake: When Bart and Edna are watching "Holes" in the IMAX theater, Edna sighs and pulls out a locket which we can see the back of. After opening it up to reveal pictures of her and Seymour, we find out its actually a triptych when she opens up another image of Seymour's mother. From the way she unfolds this extra picture, Seymour's mother would have had to be facing outwards, but in the previous shot the back of the locket was blank.
Special Edna (a.k.a. Love and Marking) - S14-E7
Factual error: During the video screening at the Teacher of the Year committee, the panel is watching the teacher who acts like Robin Williams. In the video the main judge says "Next" and pushes a button on the video control. It goes to the video that Bart Simpson sent in. How was this possible? The video control would only fast forward the video and two separate entries would not be sent in on the same tape. The committee wouldn't have spliced the footage together onto the same tape as the tapes were piled up on the desk. At the start of the scene one of the judges mentions to put in the next tape.
The Dad Who Knew Too Little - S14-E8
Other mistake: When Bart flashes Lisa's laser at Principle Skinner's bottom, Chief Wiggum can still see it even when his car is in the way.
Strong Arms of the Ma - S14-E9
Audio problem: The speech does not match the lip movements from when Bart says, "dad, do you plan to get in shape?" until Lisa says, "Uh guys, mom just left the basement." (00:12:17)
Strong Arms of the Ma - S14-E9
Audio problem: When the piano is playing Fur Elise, the keys do not move.
Strong Arms of the Ma - S14-E9
Audio problem: When Marge sprays the pepper spray at Ralph, the line she says after does not match her mouth.
Strong Arms of the Ma - S14-E9
Audio problem: When Homer says he'll eat twice as much, Bart says excellent, but that doesn't match his mouth.
Strong Arms of the Ma - S14-E9
Audio problem: When the woman who smashes the piano says "support the arts", her mouth is open, but it doesn't move.
Continuity mistake: When Homer shows his "incriminating photos," you can see that in them, he is bald. We know from several previous episodes that Homer had hair when Bart was a baby. He even has a bit of hair when Lisa is born.
Continuity mistake: When Tony Hawk and Homer compete against each other on the skateboards, Bart is on Tony's starting side. When Homer knocks Hawk off his skateboard, Bart is now on the side that Homer started on.
A Star is Born-Again - S14-E13
Audio problem: When Lenny talks about selling Sara's autograph on eBay, his lips don't match what he says.
A Star is Born-Again - S14-E13
Audio problem: When Homer tells Ned "Flanders, I'm always interested in talking about free milk. And if there's cookies involved, so much the better", his mouth is not in sync with his voice.
Mr. Spritz Goes to Washington - S14-E14
Audio problem: At the end of the episode, when the guy who helped the Simpsons pass the bill says, "I just passed a bill in three hours. Quit complaining," his mouth movements don't match the words.
Scuse Me While I Miss the Sky - S14-E16
Other mistake: Bart and Lisa overload the nuclear power plant to create a city-wide blackout so everyone can see the meteor shower. All the lights in Springfield blow out. Yet when we see the Simpsons sitting on their lawn watching the shower, every single window in their house is yellow, as if every room has their lights on. It is the only house shown with power.
Scuse Me While I Miss the Sky - S14-E16
Audio problem: In Declan Desmond's film of Springfield Elementary shown at the end, Kearney and Dolph start beating Jimbo up. After Kearney says "Sellout," he says "Yuppie" in Dolph's voice. Then Dolph says "Soccer Mom" but in Kearney's voice.
Scuse Me While I Miss the Sky - S14-E16
Continuity mistake: When Skinner opens the door to the room Lisa is in, there is a red book with white rings around it behind the door, it is easily the most noticeable book on the shelf. When the shot cuts back, the book is suddenly blue.
Dude, Where's My Ranch? - S14-E18
Continuity mistake: When Lisa meets Clara, there is a close up of her necklace. The necklace is never there in any of the other shots that we see her. Her ring, as also seen in the close-up isn't visible either.
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