Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Homer is sitting on the settee, you can see his beer glass is empty, even though a few seconds ago it was nearly full.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when the dishes are cleaned, you can see by Marge's plate there is a spoon and a fork. In the next shot, there is now only a spoon, which disappears in the next shot.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Marge has finished eating her meal, you can see a spoon on her plate. But when she is seen on the camera, the spoon has gone.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Marge opens the door to the robotic salesman, you can see there is a green mat under the door. But when the salesman puts his foot between the door, the mat is missing, and in next shot the mat has re-appeared.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when the leprechaun knocks Marge's glass over on the table when she is eating her pancakes, the glass changes position between shots.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Homer puts his spade in the ground when he is digging a hole with Bart, the spade keeps disappearing and re-appearing between shots.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Lenny and Carl are crushed by the helicopter at the tavern, the liquid behind Lenny is white but changes to brown in the next shot.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Homer, Bart, Lisa and Maggie are eating their breakfast at the table, they see Marge with a blue beard and Homer spits some food on the table. In the next shot, the food has gone, and when Homer goes to strangle Bart, it is back again.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when the Fortune Teller is speaking to Homer in her office, you can see the crystal ball is in front of her but in the next shot it has moved to the edge of the table.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Marge, Bart and Lisa are at the Fortune Teller's office, you can see the Fortune Teller has three cards in front of her on the table. When Homer arrives, the cards have gone, and later on in the scene they are back on the table again.
Little Girl in the Big Ten - S13-E20
Other mistake: Sherri and Terri are seen participating in the second grade gymnastics (as referred by the gym teacher), even though they're in the fourth grade.
Half-Decent Proposal - S13-E10
Audio problem: The lady singing at the prom doesn't move her lips when she sings, her mouth just stays open.
Continuity mistake: When Duffman says "Duffman can never die, only the actors who play him", his red briefs are missing.
Sweets and Sour Marge - S13-E8
Continuity mistake: When Homer dumps the sugar into the ocean, the hand pressing the "dump cargo" button has a yellow shirt cuff. Homer is wearing a white short-sleeved shirt.
Sweets and Sour Marge - S13-E8
Continuity mistake: At court, when Garth Motherloving tells Judge Snyder "I'll kill you while you sleep", the blue haired lawyer's suit is green. His suit is grey in all other shots of him.
Audio problem: When Homer says "He sees a thousand times worse on that animal channel", his mouth is out of sync when he says "channel."
Continuity mistake: At the end of the episode when Bart, Lisa and Maggie greet their parents Bart says that they were in the Green Room (a room where the cast of a show can relax when not performing) and comments that '[he] ate so much shrimp!' This conflicts with the episode 'Please Homer, Don't Hammer 'Em', where it is revealed that Bart is allergic to shrimp.
The Lastest Gun in the West - S13-E12
Continuity mistake: At the start of the episode, the dog eats Bart's clothes through the letterbox, but a minute later, when the dog next comes to the house and Homer opens the door, there is no letterbox.
Little Girl in the Big Ten - S13-E20
Revealing mistake: When Bart says "I don't feel so good" after being bitten by a mosquito, his lower body under the table is not visible.
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