Continuity mistake: When Homer gets thrown out of the house by Marge and Otto walks up to him, you can see that it is Smithers and Carl standing behind Homer. Yet in the next shot when you see Homers face, you can see it is now Principal Skinner and Carl standing there.
Other mistake: In the scene where the Simpsons are viewing the path of the Corvair's Re-entry, the blip representing the spaceship crosses the entire US from North-West to South-East in about three seconds. This is really impossible, even for something going as fast as a spaceship on re-entry would be.
Continuity mistake: When the person delievers 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: When Marge comes home after the bogeyman incident, Homer trips over a footrest. You can see Bart sitting there however his pyjama's are a dark green color when before they were a bright green color.
Revealing mistake: At the dinner table Homer says something like "as you know it's always been my lifelong ambition to be a blackjack dealer." During this line, for some reason they appear to have used a clip from an older episode (Season 3 / Episode 3 - When Flanders Failed) and dubbed the line over. For one thing, the animation is of a much lower standard than the rest of the episode. And perhaps even more telling, when he says it, he is holding a fork with some food on it, but from the earlier shots we know there is no food in front of him.
Continuity mistake: The man conducting the tour of the box factory tells the children that they will not be able to see a finished box, as the boxes are assembled in a separate location. However, moments earlier we saw a flat box going through a machine, and coming out finished.