Continuity mistake: When Homer puts Lisa's test on the fridge, a lot more papers appear on the fridge. (00:04:50)
Continuity mistake: After Mrs. Krabappel asks Bart to name the pirate in Treasure Island, when it cuts to a closeup of Bart we see the set of blue books have vanished from Mrs. Krabappel's desk, and a Krusty mug has suddenly appeared there. (00:03:00)
Like Father, Like Klown - S3-E6
Other mistake: When Krusty leaves the Simpsons, The Front door is just an empty space missing all the details. (00:11:20)
Like Father, Like Klown - S3-E6
Continuity mistake: At the end of Krusty's story the candles have burnt themselves out on the table. However in the next shot they are alight again. (00:10:05)
Like Father, Like Klown - S3-E6
Continuity mistake: Towards the end of the meal, Lisa has some meat left on her plate that disappears in some shots completely. (00:06:55)
Like Father, Like Klown - S3-E6
Continuity mistake: When talking about Jewish people, Homer puts mashed potatoes on his plate that sometime disappear and reappear. (00:06:30)
Like Father, Like Klown - S3-E6
Continuity mistake: Lisa says about Krusty being Jewish and Homer's wine glass swaps side on the table. (00:06:20)
Like Father, Like Klown - S3-E6
Continuity mistake: Homer asks if Krusty is going to eat his meatloaf and is reaching across the table as he does so. The camera changes and he is no longer reaching. (00:06:50)
Other mistake: When the family are having dinner and Homer has a beer, the can is grey instead of the usual red. This is rectified a few seconds later. (00:14:00)
Continuity mistake: The zipper of Burns' radiation suit disappears instantly when he's on the phone to the news station. (00:06:20)
Continuity mistake: Milhouse unwraps his present from Bart and the wrapping paper stays under the box. When he's seen a few seconds later however the paper has gone. (00:01:50)
Continuity mistake: When Bart is writing the card for Milhouse, he is writing on the right hand side of the card. This changes to the left hand side in the close up. (00:01:10)
Continuity mistake: Maggie spits a hotel out on to the board which disappears a few seconds later. (00:16:15)
Continuity mistake: When Burns hangs up the phone to Kent Brockman, his name tag on his radiation suit disappears. (00:10:20)
Continuity mistake: When the purple jam pops on the core temperature dial, it leaves some residue on the dial which has disappeared in the very next shot. (00:05:10)
Continuity mistake: When Homer is talking to Lenny and Carl about the movie his pink doughnut disappears from his hand. (00:03:45)
Trivia: In the opening sequence post Feb 2009, Marge can be seen placing items on the checkout. These include the detergent Mr Sparkle from Season 8 Episode 22, In Marge We Trust, and some Tomacco juice from Season 11 Episode 5, E-I-E-I - (annoyed Grunt). (00:00:25)
Continuity mistake: The colour of Bart's plate at the BBQ changes between shots. It goes from dark blue to light blue. (00:07:40)
Continuity mistake: When Homer and Ned are talking after Homer tries to return the BBQ, the BBQ moves place on the lawn. Note the stepping stones. (00:17:40)
Continuity mistake: At the end Todd's shirt changes from white to red. (00:21:15)
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