The Girl Who Slept Too Little - S17-E2
Continuity mistake: At the end of the episode where Homer is on the field talking to Buck the baseball player, the stadium is crowded. When Homer and Marge are leaving in their baseball golf car one second later, the stadium is empty. (00:20:40)
The Bonfire Of The Manatees - S17-E1
Continuity mistake: When Marge is in the car upset at Homer, the door to their house changes from open to closed then open again between shots.
The Bonfire Of The Manatees - S17-E1
Factual error: Alec Baldwin, as an expert on manatees, says that they are also called dugongs. Manatees are found in Atlantic coastal waters and in the Amazon and tributaries. The dugong is a distinct species found in the coastal waters of the Indian and southwest Pacific oceans.
Factual error: Moe has a double barrelled shotgun that fires four times. A double barrelled shotgun can only fire two shells before having to reload.
The Last Of The Red Hat Mamas - S17-E7
Character mistake: When Homer finds Marge's folder, it reads "Heist Plan: Burns Manson" instead of "Mansion" as it should be.
Treehouse Of Horror XVI - S17-E4
Continuity mistake: At the end of "B.I.: Bartificial Intelligence," Homer climbs onto the ceiling and turns his head in a parody of "The Exorcist." He then lets go and lands on the bed. The only problem is that his body landing doesn't match the position it would from what we see on the ceiling. If he dropped down his face should be face down, body facing up and his head would be closer to the end of the bed. When he lands, his head is near the pillows, his body facing down and his head facing up.
Continuity mistake: When the jury is "oohing" Wallace Brady, Lenny's five o'clock shadow is missing. It reappears in the next shot.
Continuity mistake: When Homer says to Bob that he tells Marge everything, his hand is by his mouth, the shot cuts and it is at his side.
The Seemingly Never-Ending Story - S17-E13
Continuity mistake: When Lisa pets the goat after it returns her necklace, her pink jacket is missing, in the next shot, she has the pink jacket on.
Homer's Paternity Coot - S17-E10
Continuity mistake: When Dr. Hibbert is about to reveal the results of the paternity test, Mason Fairbanks is wearing a blue blazer and black pants. But, when Dr. Hibbert points at him and says "This guy", he has a light blue sweater and blue pants.
Milhouse Of Sand And Fog - S17-E3
Continuity mistake: Homer's chicken pox disappear when he puts the food into the suitcase.
The Last Of The Red Hat Mamas - S17-E7
Factual error: Milhouse says he speaks fluent Italian, but his Italian is full of basic grammatical errors. He says that "insegnante" means "masculine teacher" when the same word is used for a male or female teacher. There are errors in Lisa's Italian course too ("barca piccola" instead of "piccola barca"). Despite Lisa's understandable mistakes, as she is a learner, Milhouse never corrects her (there is no such word as "idioto"). Even Milhouse's grandmother, who is Italian, makes many grammatical mistakes.
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