Continuity mistake: Britney's nails keep changing throughout the movie.
Other mistake: Right after Britney visited her mother, she goes into the washroom and cries. The lightswitch in the washroom is "off" and even if there was a window in the ceiling - it was raining outside. So how is it possible that there was light in the washroom above the bathtub?
Continuity mistake: When Mimi, Kit, and Lucy are looking out of the bathroom door at Ben right after Kit tells them that he killed a guy once. When you see Kit looking out of the door she has her toothbrush in her mouth. When they shut the door and you see them talking in the bathroom again she has the toothbrush in her hand and then puts it in her mouth.
Continuity mistake: 2/3 of the way through, in the scene when Kit is on the phone with her boyfriend, you see Ben and Lucy on the bed working on a song. Lucy is lying on the bed writing words down. The scene cuts from Kit on the phone to shots containing Lucy. When you see Lucy sometimes she is lying in a position with her head down on her arm and sometimes she is up on her elbows. The scenes are switching back and forth very quickly and looks funny because how the heck is the song getting written with her flopping all over like that?