Continuity mistake: When Napoleon drops off the drawing of Trisha with her mom, he is standing on the porch. He then says 'bye' and runs off of the porch. The next camera shot is from Trisha's mom's perspective of the screen/glass door closing. When the door is closing you can see Napoleon's reflection in the glass (he's standing right there when he should have been far away from there if he had run off the porch as portrayed).
Continuity mistake: When the bully beats up the nerd with the turtleneck and puts him into a headlock, notice the disabled girl in the red shirt that walks by them. She walks past them twice in about 6 seconds.
Continuity mistake: When Napoleon starts his dance skit near the end of the movie, his t-shirt is tucked into his pants. Shortly into the skit, the shirt comes loose in the back and on one side. A few seconds later, the shirt is once again completely tucked in. This happens several times throughout the scene.
Continuity mistake: When Summer and Trisha are handing out "vote for Summer" fliers, in the close up of Summer handing them out you can see that one of the girls has on pink nail polish. Moments later, in a shot from a further distance (yet close enough to see nails), neither girl has on any pink nail polish.