Continuity mistake: During the Bella's performance of I Saw The Sign in which Beca starts singing Bulletproof, when Chloe sings the line "like me care for you" the camera focuses on the audience and there an older woman with glasses sitting next to a guy in a blue shirt, but in the next shot the woman has moved seats, and is now sitting next to a guy in a yellow shirt in the row behind the judges' table.
Visible crew/equipment: During the Finals of the International Championships at the end of the movie. Beca is singing "Don't you forget about me" the camera cuts to Jesse sitting in the crowd with Donald sitting next to him, in a few of these shots, you can see crew in the reflection off Donald's glasses and an empty stage.
Continuity mistake: In the scene where Beca's dad wakes her up for her class her desktop is "awake" (showing her home screen). A few shots later Beca's dad walk towards it and the desktop is on its screensaver then gets bumped to awake. If the screensaver was set to activate after a period of inactivity, it would already have kicked in while Becca slept, not in the short time it's offscreen.
Continuity mistake: When Benji and Jesse are approaching the Treblemakers at the activities fair, and the Treblemakers begin singing "Whip It," The Treblemaker in a blue shirt goes from the steps on the left to the wall on the right between shots, and is back on the steps when we see Aubrey and Chloe watching them.
Character mistake: When Jesse is talking to Becca about "The Breakfast Club," he tells her that the best song is "Don't You Forget About Me" which was recorded by Simple Minds because Billy Idol didn't want to do it. Steve Schiff and Keith Forsey composed "Don't You..." Billy Idol approached them, requesting to sing the track. Steve Forsey didn't want Idol to do it for the movie, and Forsey personally chose Simple Minds. Idol was eventually allowed to record his version of the song, and it's on his album "Billy Idol - Greatest Hits" as remastered in 2001. He stayed true to the original.