Continuity mistake: Just after Bart has played the guitar in the street, he picks up two books. Before Bart picks them up they are green but they instantly turn blue when he does pick them up. Then when he gets on the bus the books have disappeared completely. (00:10:10)
Continuity mistake: Bart's guitar is lefthanded. So when the righthanded Otto plays it on the bus, the guitar is flipped over, with the little speaker on the top. But when Otto is playing it later in the garage, it is a normal righthanded guitar, with the little speaker on the bottom.
Continuity mistake: When Skinner is calmly driving the bus on the way to school, they have obviously gone past a number of power lines (you can see that they have because you can see them in the background). However, when Skinner is trying to get onto the main road, the power lines have disappeared from the background. (00:12:45)
Continuity mistake: Near the end of the episode, when Skinner shouts at Ralph to shut up, the sky outside the bus changes from nighttime to daytime between shots. (00:14:15)
Continuity mistake: When Nelson asks Bart what song he is playing as they wait for the bus, Nelson's vest is orange and his shirt is bluish. However, when the bus arrives and Nelson goes to get on the bus his vest is now bluish and his shirt orange. (00:10:10)
Revealing mistake: When Skinner starts to drive the bus, he says "Off we go". During this shot the sky is a brighter colour when you look out of the bus windows than if you just look outside the bus. (00:12:15)
Other mistake: Right before Homer and Bart head to the concert, Homer has the high-pitched ringing in his ears while Marge is talking. Then just as Homer says, "I hear you. C'mon, boy," note Selma's and Patty's skin over their scoop necklines are not colored in properly, it's the same color as their dresses. (00:01:45)
Continuity mistake: While Otto plays the guitar at 3:00 AM, the tuning keys on the guitar head are first facing down, then the guitar head flips and the tuning keys are facing up, between shots. (00:15:45)
Continuity mistake: Bart's guitar disappears when Otto starts to race to school. (00:11:15)
Continuity mistake: When the band is talking to the audience about being the 6th member of Spinal Tap, the water on the stage simply vanishes between shots. (00:06:55)
Continuity mistake: When we see Homer in the car for the 1st time since the band hit the stage, he has his hand up in the exterior shot but in his bag of crisps from the interior shot. (00:05:20)
Other mistake: At the concert when Bart throws the Frisbee into the dude sat next to him, the crowd in the background are all missing their eyes. (00:03:50)
Continuity mistake: When Marge tells Otto to get a job, the books on the bookcase by Otto change/move between shots. (00:16:50)
Continuity mistake: When Otto crashes the bus, there is no sticker on the bumper. When the bumper flies off, a sticker has appeared. (00:11:30)
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