Character mistake: At the activities fair, after Benji and Jesse walk away from the Treblemakers, Bumper says "Let's match pitch." Then sings a note. The rest of the Treblemakers harmonize, rather than match the note that Bumper sings.
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: At the second competition when Beca runs out, you can see the family walking down the hall with the mom, dad and son in that order from left to right. When Benji runs out after her, the kid is then in the middle.
Continuity mistake: In the party scene when they show Bumper and Donald, a girl with a green shirt and a leather jacket walks down the aisle. When Fat Amy approaches Bumper, the same girl walks down the aisle as if she is just arriving.
Continuity mistake: During the finals, Cynthia Rose's blazer is done up, but during the next shot, it's undone, then the next shot shows it's done up.
Continuity mistake: The water bottle to Fat Amy's right on the floor is present in 1 shot and next shot it is moved behind her much farther away.
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.
Continuity mistake: In the scene where Beca is going to do night shift at the radio station,when she is unlocking the door to the radio station she has a handbag on her shoulder but when she enters the booth in the radio station she has a backpack on her back instead of a handbag.
Continuity mistake: When Beca walks into the radio station at the beginning it says 85.7 on the window but when she walks into the studio the guy inside says 95.7 WBUJ over the tannoy.
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.
Continuity mistake: When the Treble's are singing "let it whip" in the activities fair, Donald changes from sitting on the wall, to standing on the stairs, then back to sitting.
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.