Continuity mistake: The location of the comet in the sky changes direction at several points. Whilst it's apparent size and altitude will of course change, the direction it is coming from cannot. As an example, it is seen looming over the power plant towers, but when Ned is on the hill, it's no longer there.
Continuity mistake: In the last scene when Homer is about to attempt the loop trick, it is seen several times that the Mafia guys are not standing in a straight line. In all shots they are except the one where Homer fails, and then says "Ta da,". The Mafia are shown head to head in a straight line.
Continuity mistake: When Homer is threatening to boot the Prime Minister, he is holding him still with his left arm. When the shot goes to behind Homer, he has his right arm on the Prime Minister's shoulder. When the shot cuts back to a direct view of Homer, his right arm isn't touching the PM.
Character mistake: Burkina Faso is listed as being among the Southern Hemisphere locations Bart called when Homer is looking over the phone bill; since Burkina Faso is completely within the Northern Hemisphere, it would have made no sense for Bart to call there.