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.
Colin has no visible tattoos - especially obvious in the basketball scene - but he has a tattoo on his calf when he turns the tub faucet off with his foot.
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.
In the beginning of the movie, Anna Faris's top is purple but when she goes into the bathroom it is black. The color changes back to purple on returning from the bathroom.
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 Daisy and Ally using umbrellas to fend off rain there doesn't seem to be rain. And people in the background don't act as it is raining.
When Ally is taking a shower and she suddenly remembers the name of "Valerio", and she immediately gets out to write it down, she is seen wearing skin-color hot pants. Which is odd, as Anna Faris was, in fact, totally nude.
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.
When Ally gets off the T to go to her job, she gets off at State Street Station, on the Orange and Blue lines. However, after she's been fired, she is waiting on the platform at the Porter Square Station with her box of things, presumably going home. Porter Square Station is in Cambridge, not Boston, and it's on the Red line, nowhere near State Street, or the Orange or Blue lines.
At the beginning of the film, when Ally is talking to her sister on the phone, 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.
When Ally receives a call from her mother while riding the train, she holds her iPhone upside-down.
Simon mentions that he longs for anything English, so Ally puts on a fake British accent. Simon, being British, would notice the fake accent straight away, yet accepts it as genuine.
During her shower the curtain is hung so it drapes outside the tub, which would cause water to drip onto the floor.