At the a-ca-initiation party, "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 Cynthia Rose is shown playing poker. They were playing from left to right and poker is played from right to left.
During the Bellas' semi-final performance, right after they finished "I Saw the Sign/Bulletproof" and are moving onto "Eternal Flame," look behind Beca. Ashley goes cross-eyed for a moment right after her eyes open. She re-adjusts her eyes a second later.
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.
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.
(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.
In the audition song, the guy who sings the 2nd line 'it was cool but it was all pretend' is shown already standing with the BU Harmonics group when Beca first meets the Bellas and they show her the different campus singing groups.
Twice we see the sign on the door of the radio station showing 85.7 FM while we hear it advertised as 95.7 FM. (Also, frequencies below 87.5 FM are not used in the United States.) Also, radio stations based at educational institutions in the United States are based between 87.5 and 91.5 FM.
During the activities fair, Aubrey and Chloe say that they need to recruit 8 girls. Thus, a team to be able compete in the ICCAs must have at least 10 members. However, when the Regionals are happening the team with the pink dresses standing in front of the Bellas has only 8 members present and the team on stage (the Sockapellas) has only 6 members.
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.
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.
When Jesse is backing up during their set at finals, you can see that his jacket got caught on his arm. He then shakes it off, the scene switches frames to just Jesse and Benji's face where you can see his shoulders did not move at all. Then, another frame switch and his blazer is caught on his arm again.
After the Bellas' final performance, as they're taking their seats, you can see that the extras in the audience are only pretending to applaud. Many of them don't even come close to connecting with their hands.
When Beca enters the radio station, she walks through a door that has "Studio" and other words painted on its glass backwards, so they are readable from outside the room. There is also a window treatment over the glass, but on the outside of the room, instead of the inside. This was probably done so the door could have the descriptive words and the blinds, but the blinds cover the words from where you would want to read them.
When Jesse is eating dinner with his family, they are eating next to a kitchen island with a distinctive faucet. Later Beca visits her own father in what is clearly the same kitchen but shot from a different angle; the tell-tale sign is the same island counter-top and the distinctive faucet.
The goof items below may give away important plot points.
Aubrey disappears after the closing number. After the Barden Bellas finish their song, the camera cuts away to the commentators. Before that cutaway, you can see and count all 10 Barden Bellas. Afterwards, there are only 9 Bellas as they head off stage before Beca greets and kisses Jesse. Moreover, when the Bellas are performing, all the Trebles are shown in the audience. However, after the Bellas get off the stage and Beca goes into the audience to meet Jesse, he is the only Treble left there. The rest have disappeared.