In Jenna's room on the day of her 13th birthday party, there is a framed picture of Madonna on her vanity table, leaned on the mirror. When she is 30, she comes across the same photograph, but it is signed.
Jennifer Lopez is featured on both the Sparkle and Poise magazine covers. Ironically, during the filming of this movie, Jennifer Lopez was in a serious relationship with Ben Affleck - Jennifer Garner's future husband.
The slumber party scene was forced upon Gary Winick at the producers' insistence, over his strenuous objections. He felt it cut away from the Jenna/Matt romance, which was the real crux of the film. But watching it on the DVD Directors Commentary, he then felt it was one of the best scenes in the movie.
Gary Winick wished he'd done the magic scenes and where Jenna first sees she's an adult differently. Adult Jenna first seeing herself in the mirror was a deliberate homage to Big (1988), when Tom Hanks first saw himself as a grown-up.
The blindfold Jenna is wearing when she is 13 is made out of the same material/pattern as the eye cover she is wearing when she wakes up when she is 30. The blindfold and eye cover were both designed by Cris Notti, an accessories designer based out of Los Angeles.
When Jenna first gets to Poise, her assistant says that Eminem is on the phone and needs a decision. She calls out "plain, peanut, plain!", thinking the assistant means M&M, since she doesn't know of the rapper Eminem yet
Gary Winick likened the scene between Jenna and Mark in Central Park to the balcony scene from Romeo & Juliet. Winick later payed homage to the play even further (with an actual balcony scene) in his final film Letters to Juliet (2010).
Originally Young Matt was to be played by Jack Salvatore Jr.. Test audiences reacted negatively towards him and the girl originally playing Young Jenna (Shana Dowdeswell), so the parts were re-cast and the scenes were shot again.
The scenes on Jenna's 13th birthday were shot twice with two different wardrobes. In the movie trailer young Jenna is wearing a multicolored top and has her hair down but in the movie she is wearing a blue top and her hair is up.
When Jenna announces, "I also strongly suggest we take apart our FOB, overhaul the BOB..." She's referencing magazine publishing industry acronyms which stand for "Front of Book" and "Back of Book" and refer to the short articles placed at the front and back of the magazine.
Young Matt Flamhaff listens to "Burning Down the House" by The Talking Heads at Jenna's 13th birthday party. The drummer for the Talking Heads, Chris Frantz, was also a member of his wife's group, "Tom Tom Club" at one point in his career. Tom Tom was Lucy Wyman's nickname as a 13-year-old in the movie.
The trivia items below may give away important plot points.
When Jenna wakes up and she is 30, she is terribly lost in her apartment. In order to know where she is, she quickly takes a look at some mail she received. One of the envelopes shows that the letter was sent by Sparkle. This will have great significance later in the movie.
When Jenna confronts Matt as he is dressing for the wedding, a book with the title "Time's Arrow" can be seen on the shelf beside her. The book is a novel by Martin Amis, about a man who experiences his life in reverse: the book opens as he becomes conscious as a very old man near death, and proceeds backwards through his life, ending when he is a small child.
Jennifer Garner's first scene was crashing Matt's wedding, one of the last scenes in the movie. Jenna was supposed to get to the wedding by hitchhiking, but the writers wanted to do a scene with Jenna and an adult Chris Grandy, so they made him into her cab driver.