At the "A-ca-initiation", "I Can Feel It" is selected on the MacBook but "Keep Your Head Up" starts playing and even though Justin hits keys, the play button isn't actually pushed (the play symbol doesn't change to a pause symbol).
When Beca is lying back in the park with her headphones on, she has the one labelled "R" over her left ear. A music nut like Beca would never wear headphones with the channels reversed. (On this model headphone the cord extends from the left side and the filmmakers likely didn't want the cord in the shot.)
When the girls are notified that a competing team was disqualified and that they are now going to the finals, one of the girls are shown playing poker. They were playing from left to right and poker is played from right to left.
Upon being picked up by the taxicab, Beca has four black bags/suitcases, plus her black laptop bag placed into the taxicab's trunk. Upon arrival at Barden, she grabs the laptop bag, while a yellow-shirted volunteer unloads the trunk. The bags do not follow her to her dorm, and once in the room, Beca is unpacking boxes and a brown duffel bag.
Aubrey is talking to The Barden Bellas about rehearsal times when standing in front of a white board displaying the rehearsal schedule. Aubrey says "we will practice every day for at least two hours seven days a week..." however the board behind her clearly says under Sunday: "No rehearsals - enjoy it!"
When the guys come in to announce there's no one left to audition, Aubrey and Chloe have no one sitting behind them. The shot changes and there are 2 people sitting behind them. When Beca enters, those people disappear again.
When Chloe said she just had her surgery regarding her node and couldn't sing any higher than a G#, she actually hit a mid-high C in "Just the way You are", and something higher than a G# during the finals as well.
In the finals, when the Bellas are doing the stationary walk dance in a line, Aubrey's hair is clearly let loose.
However, in the next scene, her hair is neatly tied in a bun again.
Afterwards, she then lets her hair down.
When Chloe, Aubrey and Fat Amy are fighting, Lilly attempts to join the group and is wearing a green bow on top of her head. Fat Amy pushes her out of the way and Lilly's bow falls off. When she lands in the pool of vomit in the next shot, the bow is on her head again.
When Jesse is about to start the end of The Breakfast Club, he turns one of the lamps next to Beca's bed off. Since there is a light on Beca's desk, the room should still be lit, but the room goes completely dark.
(at around 1h 13 mins) When the Bellas are singing "The Sign", a shot of the audience shows a red-headed girl with a lavender top. Three seconds later the same girl has moved and is sitting just below the judges surrounded by different people.
During the conversation between Bumper and Benji at the student fair outside, you can see Unicycle walk away but when the camera switches back to Bumber from Benji, Unicycle is back leaning against the post as if he never moved.
When the girls sit down to "say something about ourselves" Denise puts a towel over a chair for Lily to sit on because of the vomit on her back. However you can see that Lily's back is perfectly clean.
Just before Beca's attempt to make amends with Jesse, she turns a corner on the way to his room. As the shot begins, there is an extra "walking" down the hallway in the opposite direction; however it is clear that the extra was standing still as the shot began and started walking as Beca turned the corner, which looks highly unnatural.