Continuity mistake: When the camera shows the table of egg food, several flies are crawling around the sandwiches. It is highly unlikely that either the flies all at once left their meal in the short time before Napoleon moves to grab a sandwich, but we see no flies fly away when he does so.
Continuity mistake: When Napoleon, Kip, and Grandma are all talking in the kitchen, notice the bag of Tostitos next to the refrigerator. In the close-up shot of Napoleon, the bag's front is facing to where it can be read. Moments later, in the camera view from Kip's perspective, that same bag of chips is now backwards. Then in the next close-up shot of Napoleon, the front of the bag is facing forward again.
Continuity mistake: When Grandma is motorcycling at the dunes, the lines in the sand change dramatically between shots.
Continuity mistake: When Pedro is sitting at the table in the cafeteria, Napoleon comes to sit down, and Pedros's arm changes position from shot to shot. (00:34:10 - 00:34:40)
Continuity mistake: Napoleon's curls change throughout the movie, as they messed up his perm the first day, and made them too big and loose. If you watch scenes shot on the first day, they are big and loose, but the scenes that were shot later on are much smaller and tighter. This is because they fixed the perm on the second day. I saw this on a DVD.
Continuity mistake: During the first cafeteria scene, Napoleon has tater tots in a perfect pyramid on his plate. When the camera cuts back to him the pyramid is now in a pile.
Continuity mistake: When Napoleon is calling Kip, the position of the phone on the table changes between shots.
Continuity mistake: When Uncle Rico goes alone to see Deb at her studio, she's trying to decide what backdrop to use. Her hands are together and under her chin. She turns to Uncle Rico and drops her hands, but in the next shot they're up again. Then Uncle Rico's gold chain is visible, but not visible in the next shot.
Continuity mistake: When Napoleon and Pedro are eating lunch in the cafeteria Napoleon takes all of Pedro's tots and Pedro is only left with a corn dog. In some shots there is one tot next to his corn dog and in others there is none.
Continuity mistake: When Napoleon first talks to Deb at lunch, Deb has some of her sandwich on her right upper lip. In the next shot of her, it moves to below her lip, on her chin.
Continuity mistake: When Napoleon is about to have lunch at the egg farm, one egg is standing out on the top of the bowl, a second later there are more eggs. (00:41:45)
Continuity mistake: When Rex announces that his program costs, "Only $300," Kip's hands go from being on the ground to in his lap.
Continuity mistake: When Napoleon shows Pedro the picture of his "old girlfriend", the position of his fingers on the picture changes between shots.
Continuity mistake: When Pedro returns to school after being sick, Napoleon sits next to him in the cafeteria. There is a piece of food on the table next to Pedro's lunch tray, and in the next shot it's gone.
Continuity mistake: When Pedro and Napoleon are shopping for a suit for Napoleon to wear for prom, the tie knot is wide and makes the tie look wide. At the dance, Napoleon's tie knot changes and looks thinner on top, and the tie looks thinner also.
Continuity mistake: When Kip and Rico go bowling, Kip bowls and there are bumpers on either side of the lane. In the next shot the bumpers are gone.
Continuity mistake: The first time that Napoleon is seen walking out of his house to feed Tina, the front steps are in bright sunlight. When he returns a moment later, and Kip is lacing his rollerblades, the steps are covered in shade from a nearby tree.
Continuity mistake: When Napoleon is counting up his money, his watch moves further up his wrist between shots.
Continuity mistake: When Deb gives Napoleon the picture of the girl, the position of the picture in Napoleon's hand changes between shots.
Continuity mistake: When Kip runs over the plastic bowl to demonstrate its strength, watch the location of the left rear tire's valve stem. In the close-up when Kip wedges the bowl behind the tire, the valve stem can be seen near the top of the screen at the 4:00 position. When Kip walks to the front, but the van hasn't moved yet, the valve stem is now at the 7:00 position.
Chosen answer: This is never explained or referenced.