During the first scene with the English teacher, he spells "judgment" as the British spelling "judgement", even though he is American. However, this is part of a lesson about The Scarlet Letter, which is written in 17th-century English idiom.
When Marrianne and Olive become 'friends' for that one day in school, Olive hugs her in the office wearing a pink and black corset, and when it cuts to another scene in the same school day when they are yet again hugging, Olive is wearing a white t-shirt, and then for the rest of the classes back in the pink and black corset.
In the scene where Olive's mother comes into her bedroom (and she's playing the guitar) the dog on her bed is chewing on a toy. In the next cut, the toy is gone. In the next cut, the toy has returned and the dog is chewing on it again.
When Olive stands at the door claiming Mr Griffith wanted to see her, the 'A' is on the left side of her chest. When he confirms, she walks across the room with the 'A' on the right side. She sits down at the desk and the 'A' appears back on the left side.
After Olive spells out the bad word in her peas at dinner and they are discussing what her potential punishment would be, her mother's wine glass moves from being held to being on the counter numerous times.
When Olive tries to right her wrongs, she talks to Evan in an ice cream parlor to persuade Evan to tell the truth. When Evan refuses, Olive crushes an ice cream cone but, in the next shot the cone she crushed is still there.
During Olive's date with Anson, the waitress sets down their plates and as soon as she places their cutlery on the table the staff can already be heard singing happy birthday but when the shot cuts to them carrying out the cake the waitress is seen in the line right behind Todd.
After the scene of Olive and Rhiannon sitting on top of the yellow car, the voice over of Emma Stone says "The next day, things took a turn for the scandalous." yet the very next scene Olive and Brandon are both wearing the exact same clothes from the "previous" day.
At the restaurant where Olive is on a date with Anson, Olive's bib changes positions in certain scenes. One scene it is tied around her neck and in the next it looks like its tucked into her shirt, down around her chest, and then back to her neck and so on.
In one of the first scenes with Olive and Rhiannon, where they are walking down a school hallway, a mic belt can be seen on Olive's shirt. Emma Stone herself pointed this out in the DVD commentary for IMDb's benefit.
At the beginning of the movie, when Olive and Rhiannon are walking down the main street of Ojai (the street with all the arches) you can plainly see the California Highway Patrol car and the officer holding traffic at the intersection to keep traffic off the street while filming.
In the scene where Brandon and Olive are in her bedroom, the desktop on her computer shows Olive and Rhiannon from afar. However, in another shot afterwords it appears to be a closeup of the same picture. When talking with her mother when getting the gift from Brandon, the picture changes a lot, but if you watch closely, when her mother is saying her father is a little too straight, you can see her background image change, so it rotates through different pictures.
The goof items below may give away important plot points.
In the end pep rally scene, Olive grabs a band members trombone slide and pulls it outwards and pushes it back inwards creating a sound going up in pitch and then down in pitch, which is incorrect. In fact a real trombone should go lower as the slide is pulled out and higher as the slide is pushed in which would create the exact opposite sound as depicted in the movie.
In the final pep rally scene, after Olive's singing performance, when principal Griffith charges towards Olive to bring her to his office, he is for some reason wearing a different outfit from when he introduced the Woodchucks earlier in the scene.