At Jenna's 13th birthday party in 1987, they drink Diet Coke from cans with 2003 graphics.
Immediately after Jenna presents her new magazine design, Richard hugs her and tells the group they will present it to corporate "tomorrow morning." His lips clearly say "Friday morning."
When Jenna gets the umbrella and starts walking towards her bathroom in her apartment, a cameraman is reflected in the mirror.
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.
After Jenna and Lucy leave the bar, they run into Matt. As Jenna talks to Matt, his earpiece is visible.
When Jenna gets into the car and tells the naked guy to go away, a boom mic swing's reflection appears briefly in the rear of the car.
When Jenna is getting ready for the party, she puts a lot of make-up on. In the next shot, her makeup is toned down significantly.
The "six chicks" at Jenna's party give her 7 jackets.
In 1987, young Matt's body pad (to make him look pudgy) is visible when he turns his head to talk to Tom Tom.
When the Michael Jackson: 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 Matt shares his Razzles with Jenna in 1987, the package is blue. In 1987, Razzles packages were white.
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 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 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.
When Jenna goes to her party at 27 Wall Street, she is shown standing through the window going north into Times Square. 27 Wall Street is south of Times Square, so she would not have been going north.
When Jenna is at the sleepover with the younger girls, the Milano packet on the right switches from the front of the bag to the back of the bag between shots.
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.
During the Michael Jackson: Thriller dance, Matt stops dancing in the long shot. In the close-up, at the distance shot, he's still dancing, then stops.
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.
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.
At the party where they dance to "Thriller", Jenna's hair changes from curly to straight and back to curly.
At the first Poise meeting, the camera shows Jenna writing her name on the top of her of her legal pad, a few seconds later, she writes Matt's name down for her secretary, Arlene, and you can see the writing has changed .
When Jenna jumps in the cab to get to New Jersey toward the end of the film, she throws her sweater in the cab first and then climbs in. When she exits upon her arrival in Jersey she leaves her sweater in the cab as she has nothing in her hands when she starts running.
When Matt visits Jenna's office while Lucy is there, he begins speaking to who he believes is Jenna, but can't see because she's facing the other. When the camera changes, you can very obviously see the top of Lucy's head. Since Lucy is blonde and Jenna is brunette, Matt would never mistake Lucy for Jenna even from behind.
When Jenna is on the street, about to go to Matt's apartment before the wedding, her hair is parted on the right. When she is at the apartment, her part is on the left.
Jenna's school picture does not match her face when the photographer took it.
When the DJ puts the Thriller LP on he places the needle on to track one. Thriller is actually track 4. Then the song starts immediately at the spot where the 45 begins and not the spot where the LP version begins. In other words they eliminated the entire synthesizer intro. At least they got the label design correct (Epic).
Jenna says " You had a nose job!? " before Lucy finishes telling her she did.
The picture of Jennifer Lopez on the wall changes from perfectly straight to very crooked between shots.
Lucy starts to open her purse in front of the bar. When she turns, her purse is still under her arm.
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.
While at the party to promote Poise Magazine, the level of Jenna's drink changes repeatedly between shots.
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.
When adult Jenna is in Matt's apartment she yells out "I want a fluffy pillow". Matt gets caught off guard by the outburst, backs up and bumps into the wall, then apologizes to the wall saying "oh sorry".
When Jenna and her boyfriend get back to his apartment, a basketball is on the couch. In the next shot, when they sit on the couch, the basketball is gone.
When Jenna throws the paper at Arlene in the meeting, Lucy is drinking from her cup. But in the next shot, when she looks at Jenna, her cup is back on the table without any time to set it back down.
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 and Lucy are talking on the street before going up to the office for the first time, the people walking by behind them jump around between shots.
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.
When Matt is taking their picture at the end, his hand position on the camera changes between shots.
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 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.
Lucy says that she and Jenna both had nose jobs, but in the end of the film, when Jenna has changed her fate, her nose looks the same.
Assuming the present-day sequences take place in 2004 (the year of the film's release), then the sequences of Jenna at age 13 would be set in 1987. However, the clothing fashions worn by the kids are years out-of-date and the "current" music they listen to (e.g. Thriller) was released in the early '80s.