When the adult Jenna sees her office the first time she sees her photo with her boyfriend on the wall. Both Jenna and her boyfriend are wearing the exact same clothes which they wear on the night when Jenna and Lucy run into Matt and his fiancé, even though that scene happened later in the film. Even Jenna's hairdo is exactly same.
When Richard is talking to Jenna and Lucy at the party, a woman in a blue dress and a man walk behind Richard, in front of the DJ the other side of the room. Several shots later, in a shot of the DJ stand, the couple is still crossing the room to leave.
When the Thriller song starts, the people slowly go to the dance floor. Two of the very first dancers are a guy wearing a pinkish full sleeve t-shirt and a girl with flared sleeves. A couple of shots later, the same couple are shown again as going towards the dance floor.
When Jenna realizes there is a man in her bathroom, she grabs the umbrella and undoes the strap around it. In the next shot, the strap is attached again. In the shot when she opens in front of him, it's unattached.
When Matt gives the Dream House back to Jenna, a small amount of wishing dust is stuck to the side wall. When Jenna sits in the backyard and the wind starts blowing, a lot of wishing dust is on the roof.
When Jenna hears the cell phone ring for the first time in her apartment and goes to investigate, we see a sofa table with some umbrellas attached, and a jacket draped over a box with the box visible to the left. Then, as she looks back, the box has changed position and is more to the right, buried beneath the jacket.
When Jenna and Lucy are standing outside the club where they have just had drinks, Lucy pulls out a cigarette as Jenna is talking to Matt. She is holding it in her right hand and as she says she is going back inside she puts the cigarette in her left hand with her lighter. She turns to walk back in and the cigarette is back in her right hand.
When Lucy is telling Jenna how to act outside the Poise Magazine building, the same extras walk past more than twice each. Also, there are people walking behind Lucy that we don't see in the close-ups of Jenna, even though they are standing next to each other.
In the beginning of the movie when Jenna is in the closet, she pulls out the tissues from her shirt, and they land in separate places on the floor. After she pushes Matt out and tells him she hates him, the tissues on the floor change position.
When Jenna is talking to Matt while sitting at the counter in her home, before the phone rings, there's a Diet Coke can turned to where you can see the Coca-cola logo. The camera switches angles and then back, the Diet Coke can is now completely turned around where you can not see the Coca-Cola logo, but instead the back side of the can.
Jenna is told that Matt's address is 212 Bank Street. As Jenna walks through the Village to find him, the camera pans up to show her approaching the corner of Bank Street and 4th. She walks on Bank, but turns onto 4th to enter Matt's building.
When Matt walks Jenna to her apartment, the sign on the door says 'All deliveries to 12th Street entrance.' Yet, the limousine takes her to the party on Wall Street via Times Square - thirty blocks out of the way.
The goof items below may give away important plot points.
While Jenna prepares for her presentation and when she confronts Lucy in her room, her hair is quite loose and parted in the middle. When Jenna comes out of the Poise building after Lucy tells her that she has found envelopes of Sparkle amongst her stuff, her hair is tightly parted on her right side. When she rides Chris Grady's cab, her hair is fairly loose and parted in the middle again.
When Jenna is going home from the city, she takes the Metro North Railroad. When she goes to break up Matt's wedding, she tells the taxi to head towards the George Washington Bridge into New Jersey. Jenna and Matt are next-door neighbors, and the Metro North Railroad goes to Connecticut. To get to New Jersey, she would need to ride NJ Transit.
At the wedding when Matt takes a picture as he and Jenna kiss, the flash fires on the camera, but the framed photo (presumably the very same, as suggested by the transition in the film) is taken without flash as shown by the contrast in lighting on their faces and the sunlight from behind them.