Continuity mistake: When Homer is at the retirement party, there is a far shot of him (when Smithers talks to Mr. Burns) and he isn't wearing his jacket. In every other shot Homer is wearing his jacket.
OL1V3R666
Continuity mistake: When Homer says, "You can't take our donuts," his hat is red. But in the next shot and in every other shot of Homer, his hat is blue. This is pointed out on the audio commentary on the Season 4 DVD. (00:12:35)
Continuity mistake: When the family is at dinner, there is a top down view which zooms in on them from above. In this shot there is no newspaper on the chest (to Homer's left). In the next shot of Homer a newspaper has appeared.
Continuity mistake: Willie rips off his entire shirt before he fights the wolf, but in the next scene, when he is wrestling the wolf, he is wearing a torn-up shirt.
Continuity mistake: After Bart is attacked by the wolf and goes back into class, the banner above the chalkboard that has the alphabet on it is blue. Every time we see the banner after that it is yellow. (00:18:40)
Lummie
Continuity mistake: When Mr Burns draws the moustache and horns on the T.V. screen of Marge, they just disappear in the next shot when he starts to draw the bees.
Continuity mistake: When Bart comes back into the classroom after the wolf attack, Ms Krabappel is writing on the black board and there is quite a bit of writing up there. After we see Bart the shot goes back to Ms Krabappel and all the writing has gone.
Lummie 
Continuity mistake: Mr. Burns gives Marge a present when she starts her new job. The present is light purple with a red ribbon around it. Seconds later though, the present turns to orange with a red ribbon around it. (00:11:45)
Hamster 
Continuity mistake: When the wolf runs out of Krusty's studio, the woman who described it changes from having two gloves on to one glove on.
OL1V3R666
Continuity mistake: When Homer and Marge come home from the dinner, Marge is wearing a fancy dress. But when Bart falls out of the house right after Homer walks in, she has her typical green dress on.
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