In Jenna's room, on the day of her thirteenth birthday party, there is a framed picture of Madonna on her vanity table, leaned on the mirror. When she is thirty, 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 Director Gary Winick at the producers' insistence, over his strenuous objections. He felt it cut away from the Jenna and Matt romance, which was the real crux of the film. But watching it on the DVD Director's 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 thirteen, is made out of the same material and pattern as the eye cover she is wearing when she wakes up when she is thirty. The blindfold and eye cover were both designed by Cris Notti, an accessories designer based out of Los Angeles.
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.
Gary Winick likened the scene between Jenna and Mark in Central Park, to the balcony scene from Romeo and Juliet. Winick later payed homage to the play even further (with an actual balcony scene) in his final film Letters to Juliet (2010).
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.
The scenes on Jenna's thirteenth birthday were shot twice with two different wardrobes. In the movie trailer, young Jenna is wearing a multi-colored 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 F.O.B., overhaul the B.O.B." 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.
When Jenna thinks there's an intruder in her apartment, she grabs an umbrella to defend herself. When her dad hears the banging from the basement (caused by Jenna) he also grabs an umbrella for self-defense.
Young Matt Flamhaff listens to "Burning Down the House" by The Talking Heads at Jenna's thirteenth 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 thirteen-year-old in the movie.
Academy Award winner Brie Larson (for Room (2015)) can be seen as one of "The Six Chicks" in the scenes at school, and Jenna's party, and also has a line in the first one these scenes. Later in the movie you can see in a yearbook that she is no longer part of the group, replaced by Jennifer Garner's character as a teen
Jennifer Garner, Mark Ruffalo, and Brie Larson have been in Marvel-based movies. Garner played Elektra Natchios in Daredevil (2003) and Elektra (2005). Ruffalo plays Bruce Banner, a.k.a. the Incredible Hulk, in the Marvel Cinematic Universe. Larson plays Carol Danvers, a.k.a. Captain Marvel, in the Marvel Cinematic Universe.
The trivia items below may give away important plot points.
When Jenna wakes up, and she is thirty, 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.
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.
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.