Follow the Leader: A lot of elements from later light novels like Shakugan no Shana and Haruhi Suzumiya (as well as other media, as noted in the main page) can be found in this series: Urban Fantasy, teenagers with supernatural powers fighting out of highschools against evil organizations, and wholly supernatural creatures posing as students and romancing other students.
No Export for You: Seven Seas published the first three and sixth books before stopping due to poor sales (though thankfully, book 6 is a prequel to 1 and 2-3 is a self-contained story arc, so it's not like they ended on a cliffhanger). It was only after that the 2019 anime series was announced that Seven Seas promptly announced that they would release books 4 and 5, previously skipped in the 2007 print. There are currently no plans to go beyond Volume 6 (the Japanese publications are up to Volume 23 as of the end of 2019). Due to this, a Fan Translation effort is underway, beginning with Volume 7.
Schedule Slip: Since 2003, the series kept to a very consistent publishing schedule of a new volume coming out every one to two years (prior to 2003, multiple volumes were releasing a year). This pattern was broken after Volume 23's release in May 2019, as Volume 24 would then take over 4 years to release in September 2023.
What Could Have Been: The King of Distortion was going to be the final chapter of the series, which is why the next book, Boogiepop at Dawn, is a prequel (and why Boogiepop finally smiled in Distortion, finally doing what the first novel's title claimed it never did).
As stated in the novels' afterwords, Kadono himself had a lonely, meaningless highschool life back when he was a student, which can be noted in the dialogues between Keiji and Boogiepop. The Shinyo Academy was inspired by recurrent dreams he had as an adult, which he believes reflected his inner desire of returning to his teenage years and living a truly fulfilling young life. For his part, Boogiepop's trademark silhouette seems to have been inspired by an instance in which the young Kadono saw a Boogiepop-shaped shadowy mirage in midst of a crowd. Adequate, isn't?
The concept of Imaginator (and Seiichi Kirima's notes about it) is based on Kadono's reflections about how imagination and positive thinking are not always a good thing, especially whenever people, like him in this case, is forced to be positive without genuinely feeling like that.
Creator Backlash: The original artist of the light novels, Koji Ogata, was angry that the production team of the 2019 anime never called him and instead did all sorts of re-designs to the characters without his knowledge. Apparently, it was all a case of miscommunication between Kadokawa and Madhouse, which made the latter believe that Ogata had given his blessings to the designs and give them license to proceed. He eventually relented after they apologized to him.
Missing Trailer Scene: This whole trailer of the series is composed of scenes that never happen in the anime, including a character showing off powers he doesn't have in any continuity.
At some point, there was the idea of placing a single main character that would be involved in all of the events of the series, so it could be easier to understand, but this was quickly discarded because the result would change too much the story and it would not be Boogiepop anymore.
Aoi Yūki auditioned not only for Boogiepop, but also for Nagi and Kazuko. Ultimately, however, it was felt her performance as Boogiepop and Saori Onishi's as Nagi were the best for their characters.
, beginning with Volume 7.