Viola's hair color changes throughout the movie. At the beginning it's a lighter brown with blonde highlights and later in the movie it is a darker brown. The color switches back and forth between the two throughout the movie.
When Viola/Sebastian just went to her/his dorm for the first time she/he puts all her/his stuff down on the bed including the soccer ball. Then when the guys see the tampons fall out of her/his boot, you can see that there is no soccer ball on the bed behind Viola/Sebastian.
After Malcolm gives Olivia his flyer for his missing tarantula, he walks away and gives more people the flyer and keeps going. In the next scene, he is back where he was before, giving the same people another flyer.
After Viola breaks up with Justin she throws the ball at his head and you can see that it hits him and he tilts his head backwards, but when the camera changes view his he is straight back up and it is clear that these are shot at different times
In the débutante scene, Viola and Duke are kissing. First his hand is straight down, hanging along his side. Then the picture switches for one second and his hand is around her neck. Her arms go from being at her side or somewhere around there to wrapped around him as well.
Cornwall won the toss and stayed in the goal with the side of their supporters, however in the second half when Cornwall scores a goal, Justin takes a flip off the goal post and the Cornwall supporters are seen cheering behind him! Illyria supporters should have been seen since the goals would have changed in the second half. Later on in the movie when Duke and Justin are about to fight, Justin is on the goal side with the Illyria supporters.
When Viola's mother is pushing through the crowd at the carnival, at first you see her back and she pushes all the way to the front, and then you see her from the front and she is still behind multiple people. Not only that, the crowd changes and it's different people.
When Sebastian first leaves the head master's office and bumps into Olivia, all her stuff falls on the floor. When they are picking the stuff up the black folder is on top but when the head master goes back into his office Sebastian picks up the black folder off the floor.
When Viola is being Sebastian, and she is sitting at the lunch table with the other guys there is an apple juice bottle and a Pepsi bottle. The apple juice bottle moves to various spots through out the lunch scene.
When Sebastian calls Monique and leaves her a message claiming they need to talk, he says "i was thinking and, we really need to talk." later on when Monique listens to the message we hear the whole thing and he never says "and" once.
In Monique's scene, Viola turns around and her ear is sticking out through her hair. The next shot is of the back of her head, and her ear is slightly covered up, but still visible. The next shot is her face, and you can just make it out if you look closely. In the next shot, again of the back of her head, her ear is totally covered up with her hair.
When Viola walks into the tent to talk to her mother at the carnival, there is a wooden pot with sticks in it, in one scene its directly behind Viola, the camera then changes to the mother and when they go back to Viola the pot is then at least 5 meters behind her and there is a clown behind her too.
When Viola is playing in the Cornwall game, right before she reveals she's a girl, it is obvious that she doesn't have her fake eyebrows on. When she reveals she's a girl, you can see her taking the fake eyebrows off.
When Viola is following various men to imitate their walks, a magazine stand for Dose is clearly visible. This is a Canadian magazine which was only available in Vancouver, Calgary, Edmonton, Ottawa, Toronto, and Montreal.
After one of the teams scores a goal in the final soccer game, they are seen putting the ball back into play by way of corner kick; In a real game of soccer, after one team scores a goal, the other team puts it back into play with a kick-off: Kicking the ball from the center spot on the field, used to start the game and the second half.
Near the end of the Cesario's pizza parlor scene, where Viola's friends pretending to be old girlfriends of Sebastian, on the trivia pop-up track it says that in the original story Olivia disguises herself as a man named Cesario. In fact it is Viola who pretends to be Cesario. Olivia is the Duke's love interest.
In reality, débutante escorts are chosen almost eight months in advance and there are teas and manners sessions throughout those eight months. There is no way that the characters could have switched escorts so late in the story and Viola would have never been able to be an actual débutante without successfully fulfilling her year-long requirements. The movie shows her joining the débutante process two weeks before the ball.
In the weight room, when Duke and Viola are talking about Duke asking Viola out, Olivia can be seen in the corner of the shot on a machine, watching. In the next shot, someone who looks like her is on the treadmill. In the commentary, the actors and director mention that this was a different girl, and they didn't know why she was dressed the same.
The real Sebastian is subbed off in the first half of the football game because he is playing so badly, but then Viola (disguised as Sebastian) is subbed back on in the second half. In high school soccer, you can substitute as much as you want and return to the field after being removed.
Viola takes her brother, Sebastian's, place by posing as him for 2 weeks at Illyria while he is in London. Viola is enrolled at a different school, Cornwall. No explanation is given as to how Viola is able to stay enrolled at Cornwall and get away with missing 2 weeks of classes. Multiple absences would result in a phone call to the parent/guardian and disenrollment would require their authorization.
When Viola acts as Sebastian, she must wear an ace bandage to hind the fact that she has boobs. However when she reveals the fact that she is a girl at the Cornwall/Illyria soccer game, she lifts up her shirt to show that she has boobs, but there is no ace bandage.
In the carnival scene when Justin and Duke are fighting, the tip of Justin's underwear can be seen over his pants. It changes from white to black, and then back again to white in between shots. It is pointed out in the pop-up trivia that his stunt double was wearing black underwear.
Towards the beginning of the movie, Viola (as Sebastian) is called to the principal's office. She then looks in his filing cabinet for her brother's school files and when she hears the principal open the door, she slams the cabinet door shut, catching her breast-reducing wrap in the cabinet. When she goes to slam the door, you can see Amanda Bynes' eyes look directly at the wrap, making sure that it is in the cabinet well enough to get caught.