Continuity mistake: During the wedding, when seen from behind Reverend Lovejoy, there is no podium in front of him (although his hands are poised in mid-air as though one should be there). In the reverse shot, the podium has magically reappeared. (00:20:00)
Continuity mistake: When the person delivers the animation cell, his skin changes from white, to brown and then back to white. When Bart looks through the letterbox to see who was there, his skin is white. When he gives the box to Bart, it is brown. When he hits Bart in the face and says "that's for keeping me waiting", it is white again.
Continuity mistake: The cake that Marge bakes for Homer is initially white, but pink icing instantly appears on top between shots.
Continuity mistake: During the wedding you can see in the background Apu and Sanjay but Sanjay's skin is yellow.
