Continuity mistake: When Sebastian (really Viola) is trying to impress the guys at Cesario's, she uses her friends as the plan. When her second friend, Yvonne, comes over and starts to walk towards Sebastian (Viola),right before she hugs him, she is really close, but when she actually hugs him she is further back then before.
Continuity mistake: In the carnival fight scene, right after Justin throws a cheap shot at Duke, they fight and fall off the stage. Justin's boxers are black. As they continue fighting, you can see multiple times that Justin's character is now wearing white boxers. The black boxers were his stuntman's. (Confirmed on DVD special features).
Continuity mistake: In the scene where Viola is telling her mom she will be staying with her dad for two weeks, her shirt changes. When she's facing away, you can see the white tank under the blue shirt, but when she turns and walks into the dining room to talk to her mom, you can't see the white shirt. When she hugs her mom and leaves the house, the white tank is visible again. She doesn't adjust it in the scene.
Continuity mistake: At the carnival, when Duke and Justin get into a fight and Viola jumps on them, the three of them fall into a popcorn machine. Viola's mother then comes running through the crowd to see what the commotion is. There is a shot from behind her shoulder of her pushing through the crowd into the clearing, then the next shot is of her from the front, where she is pushing through that crowd into the clearing again. The people in the crowd also change.