When two of the guys that Paul shares a room with, about half an hour into the film, argue about whether a girl with a pierced tongue is a lesbian, one of them calls the other one "genius" sarcastically... but his lips are clearly saying "idiot".
In the scene where Adam, Chis and Noah are walking down the street talking about how to "Resolve this Paul issue," Noah says "I mean I try to get along with everybody but that guy's got no personality." His lip movement doesn't match up with the audio at the end of the sentence.
In the laundry room scene between Paul and Chris, Chris pranks Paul by grilling him about whether he was sleeping with his girlfriend. After fessing up to the prank, Chris says, "I'm just playing with you," but his lips are clearly saying, "I'm just fucking with you" (presumably to retain the PG-13 rating as there is another F-word mention later in the film).
Although Dora may have simply been lying, Rubin Hall is in fact not an all-girls dorm, but rather is a co-ed dorm like every other NYU residence hall. Presumably Paul would know this. (There's also mention of a fictitious Hunt's Hall, but this is allowable.)
Dora convinces her obviously over-protective mother that she is spending the night with a friend in the "all-girl's dorm." Later, she basically moves in with her professor-boyfriend for what seems like several nights without even a thought of what to tell her mother.
When Paul gets pizza, flowers and a movie, we see him getting the pizza first. As he picks up the pizza box, he has a plastic green bag in his hand (presumably containing the video), but next we see Paul at the video store, where he is pizza-less. He rents a video, which is put into a green bag and given to him. (This makes sense, since he would probably get the pizza last so it would be hot.)
When Paul is approached by Noah who shows him the party drugs, Paul pretends to put them back in Noah's pocket to hide them from onlookers. Instead of doing so he goes to hide them up his sleeve so he can throw them away. When Noah leaves, he pulls the drugs from out of his sleeve. But before this, when he does the motion to put the drugs up his sleeve, his hand is clearly empty-with no sign of the drugs.
When Dora and Paul are sneaking in to the second act of a show with the smokers, they are seen watching the song "Willkommen" from the show "Cabaret". The song "Willkommen" is the very start of the show, welcoming the viewers to the Cabaret, and would not be seen at the beginning of act two.
Though they were supposedly born during the course of Paul and Dora's outing, the kittens they find when they return to the animal hospital are clearly at least six weeks old. They have long hair, their eyes are open, and they are able to move around very well on their own. True newborn kittens are basically helpless.
Though this was probably done for our benefit, while Dora is running for the "11:50" train, we hear a voice over the P.A. system say that particular train is leaving and from what track it's leaving from. Announcement are not made for departing trains in Grand Central Station.
After seeing the musical "Cabaret", Dora sings "The Best Things in Life are Free". The song is clearly meant to be her rendition of a song from the musical, but this song has never been featured in "Cabaret". The song from the musical is actually "Tomorrow Belongs to Me".
While some have claimed that the fountain in Washington Square Park is only on during the summer (contrary to the fall and winter shots), an NYU student has confirmed that it also runs during the school year.
At the hospital, Paul is capable of giving Dora Diamond's full name to the doctor. But he was told Dora's first name only by her a few hours before, when he invited her to the Everclear concert. He's not supposed to know her last name at this moment.