When Napoleon is waiting in the van for Uncle Rico to take him to the dance, he looks at his watch. The time and date displayed is 5:40 p.m., Thursday 7/17. (They may have forgotten to change the date on the watch -- how many school dances are in July?) After he starts running down the road, he stops to look at his watch and the time and date says 5:54 p.m., Monday, 7/21.
When Napoleon is yelling at Rico to tell him, "Grandma says to go home 'cause you're ruining everyone's lives and eating all our steak," the door behind Napoleon is alternately open/closed between shots.
When Napoleon is calling Kip, the position of the phone cord wrapped around Kip changes. First, it's wrapped around his body. In the next shot of him, the cord is merely by his shoulder; then it switches to being wrapped around him again.
When Napoleon is drinking water from the fountain and the boy is being picked on, a girl with a crooked leg and a red top walks by once and exits. When Napoleon stops drinking she is at the beginning of the hallway again.
When Napoleon asks Don for a "Vote for Summer" button in the crowded hallway, there are varying amounts of people standing in front of the lockers, anywhere from a large crowd when it's handed to Napoleon, to almost no one when he throws it.
When Summer and Trisha are passing out fliers and buttons, a pair of guys walk by, one wearing an FFA jacket, that Summer tells Trisha not to give buttons to. After the camera cuts back to the girls from Napoleon and Pedro, the same pair of guys can be seen walking by again.
When Randy starts grabbing the kid's neck in the hallway, the kid immediately reaches into his pocket. After the camera cuts back from Napoleon and the drinking fountain, the kid's hand is on his neck, and then he reaches for his pocket.
In the scene following Pedro's scolding from the principal regarding the Piñata, and Pedro and Deb are chatting on the steps - a crew member wearing jeans and a white t-shirt is clearly reflected in the glass doors behind them.
When Napoleon asks if there is anything to eat, Grandma tells him to "make himself a dang quesadilla." She pronounces it wrong - it's supposed to be pronounced "kay-sa-dee-ya." However, this was intentional.
Just before Grandma leaves, she tells Napoleon and Kip that she will be back the next day. The audience can see that there is a yellow teddy bear magnet on the refrigerator with nothing underneath it. The very next day, when Napoleon gets home from school, there is a note on the refrigerator that reads, "Napoleon, Don't forget to feed Tina. Love, Grandma :-)" By this time, Grandma has already had her accident and was in the hospital, which means that she never would have had the opportunity to come home and leave the note on the refrigerator.
When Pedro jumps off the ramp with his bike, you can clearly see at least 3 blocks holding the ramp up, including one in the middle. When Napoleon jumps it, the middle block is missing so that the ramp will break.
The goof item below may give away important plot points.
In the ending sequence, Pedro is seen with a cake that reads 'Presidente Pedro! Felicidades!' The upside-down exclamation points, which should precede the words, have been left off by an Idahoan pastry chef ignorant of Spanish punctuation.