When Ally's mom confronts her and Daisy with the magazine article, the featured photo shows Ally (in a green formal gown) and Jake Adams (in a tuxedo) at the gala, which took place after this scene, not during it.
When Colin asks Ally, "who could have possibly been better than Jake Adams?" he has a piece of pretzel in his hand. When the camera zooms out he doesn't have it anymore and when it zooms back in he has the same piece again.
Approximately 17 minutes into the movie, just after the musical song with opening credits of What's Your Number? when Ally is about to toast everyone in the club, she is holding a half empty shot glass. Then it shows she has a full glass right before she takes the shot.
When Ally and Daisy are at the cake tasting, Ally is eating a piece of cake. As Ally is shoving the piece into her mouth, Daisy stops her. When Daisy stops her, there is half a piece left on the fork, but in the next shot there is less than a bite left.
When Colin sends Ally a picture of him in her bathtub, his teeth are obviously clenched and his head is facing slightly right. When Ally watches the picture, his mouth is open and his head is facing slightly left.
When the girls are playing their "numbers" game in the bar and Ally tears her piece of paper, the 2nd half is different shapes in all shots, but when they put the 2 pieces together, it's almost straight tear down the middle. Perhaps, the paper was perforated, like postage stamps are or used to be (at the corners).
Near the beginning of the film, after Ally sleeps with her ex-boss, Colin knocks on Ally's door and hands her a newspaper which she throws away with her left hand, but in the next shot she uses her right hand instead.
At the beginning of the film, when she is talking to her sister on the phone and she is wearing a red coat, she is walking down Salem St. in the North End. In the next scene she comes out of the State Station in Downtown Boston. The direction she is walking in is completely the opposite direction to any MBTA station close to the North End. She should be walking the other way around if she wanted a train station; the "Haymarket" Station.
When Ally is on the train after she got fired, she has a plant in her box which the lady, on her right, looks at. Ally places the box next to her, on her left and picks out a magazine, then gets a phone call from her mum, after Ally hangs up and the camera pans out a little, the plant is now in the lady's trolley.
After the wedding, when Ally is driving the blue car, the speedometer and RPM meter of the car are both on zero. Perhaps, the speedometer & odometer are out of order, from reckless driving (before the movie filming occurred) or they were removed from the automobile for another reason.