Author's Saving Throw: Riordan has admitted in a 2007 blog post that he provided the explanation that Athena has "brainchildren" to satisfy readers who wanted to know how Athena, a virgin goddess, could have kids. He also confesses that the real reason why he made Athena able to have kids was solely because a demigod child of Athena was too cool to not include.
God Never Said That: Most fans are convinced the Stoll brothers are twins, even though both Percy (in The Sea of Monsters) and Rick Riordan (on Twitter) have stated that they aren't.
Promoted Fangirl: Viktoria "Vika" Ridzel, also known online as Viria (see, her Tumblr and DeviantArt), is renowned for her fanart of PJO characters to the point where they were considered better, with even Rick Riordan giving her his full support. In 2017, Ridzel was commissioned by Riordan to provide official character art for his website.
Write Who You Know: As part of Written for My Kids, Riordan specifically wrote demigods as being afflicted with ADHD and dyslexia so his son and other similarly affected kids could relate. Specifically, the story first originated as something he told his son after said son asked him to "make new stories" about Greek gods.
California Doubling: The Lotus Casino scene takes place in Las Vegas, but its interiors and exteriors were filmed in Vancouver.
Dawson Casting: The titular character's sidekicks fall under this even after bumping their age up from the novels. In the book, Annabeth is 13, while in the movie she's 16, and actress Alexandria Daddario was 24 at the time of filming. Similarly, Brandon T. Jackson was 25 also playing a high schooler. This is even more jarring considering that in their most prominent roles coming into the film, they were already playing adults — Daddario in White Collar and Jackson in Tropic Thunder. This sequel was filmed three years later, with Logan Lerman now in his early 20s as well. Then again, Grover is 28 during the events of The Lightning Thief (the satyr equivalent of a 14-year-old).
Disowned Adaptation: Rick Riordan dislikes the movie adaptations of his books, and has gone on record about it numerous times, which is why he would be directly involved in the Disney+TV series adaptation of his books.
Upon being told on Twitter that a teacher was planning to show the movies to her class, Riordan posted a form letter on his blog that students could give their teachers, telling them that the movies were not just a waste of good classroom time but cruel and unusual punishment to inflict on kids.
When he was informed that the Disney+ version of the first movie censored the Lotus Casino scene, he stated that it would have been better if they censored the whole movie, just showing "two hours of blank screen"◊.
In further responses to fans on Twitter, he also admitted to never watching the finished version of the first movie (his disappointment in the script preventing him from doing so), and whole-heartedly shot down the prospect of The Heroes of Olympus being turned into films.
He did say, however, that he found the movie's take on Smelly Gabe's ultimate fate to be Actually Pretty Funny. Evidently, he liked the scene to allow [[the series to do its own version of it.
Dyeing for Your Art: Alexandria Daddario dyed her hair blonde for the second movie, as does Kristin Stokes for the musical. James Hayden Rodriguez (Luke in the Lortel run, the national tour, and the cast album) briefly did this as well, before returning to his former shaved look.
Enforced Method Acting: Uma Thurman actually worked with a snake wrangler to hold and interact with several live snakes. This helped her understand how Medusa would feel with snakes growing out of her head.
Executive Meddling: According to Riordan himself, none of the people who worked on the movies bothered listening to him (which is easily seen if you read the emails he sent to the execs that he himself leaked and compare them to the final product). It's one of those cases where the entire movie could count, thus why it's not a very faithful adaptation.
Follow the Leader: Sea of Monsters has Kronos as the main antagonist despite him spending the entire book slowly reforming in a sarcophagus. Kronos' "Titan made of lava" appearance is also reminiscent of his appearance in Wrath of the Titans.
Suppressed Mammaries: Alexandra wears layers of shirts to portray a girl in her late teens after her more adult roles in White Collar and True Detective, the latter of which especially had shown her to have grown quite voluptuous.
The St. Louis Arch sequence was in the original script draft of the first film, but the Chimera was considered "too weird-looking" and unrelatable for the audience to connect to and was thus replaced by the Hydra.
Other monsters created for the second film (but never used) were a Lydian Drakon, Boarman, and Agrius.
Juniper was supposed to have an Adaptational Early Appearance and be played by Mikayla Midgett, but her scenes were cut.
and DeviantArt
