Continuity mistake: At the very end of the episode, when Homer is being carried off by the crowd, you can see that the crowd carrying Homer, the people are small, so you'll have to look closely, but right before the credits, Marge's necklace changes from red to white, back to red and then finishes with white all in one shot.
Continuity mistake: When Bart is standing on the bench, Jimbo and Milhouse are standing in the front row, on the right side of the screen. But when there's the closeup (a second later), there are children shown in front of Jimbo and Milhouse and the girl with the purple hair is gone.
Continuity mistake: When the Stonecutters are singing "We do", the position of several characters, particularly the Martian and Chief Wiggum, change randomly as the camera angles change. The Martian starts at the end of the table on the right side, with Chief Wiggum two seats to the left of him. In later shots, the Martian, then Wiggum are both switched to the same position, midway up the left side. Then, when the song ends, Wiggum has disappeared, and the Martian remains in this new position.