Dawson Casting: The entire Host Club boys were cast with actors in their twenties at the time of filming of the live action's first episode. Yes, this includes Honey, whose actor, Yudai Chiba, is honestly the youngest at 22, followed by Tamaki (Yusuke Yamamoto, 23), Kyoya and Mori (Shunsuke Daito and Masaya Nakamura respectively, 25) and topped out by the Hitachiin twins (Shinpei and Manpei Takagi who were 25 but fast approaching 26). This is, however, averted by Haruhi herself, who really is The Baby of the Bunch for having 16-year-old Haruna Kawaguchi in the role, and Nekozawa, as Ryo Ryusei was 18. For the recurring characters, this is generally averted, except for Ayame Jonochi, whose actress was 20.
The film added some more Dawson Castings with Tomohiro Ichikawa playing Kuze at 20 and Mariko Shinoda playing Michelle at 25.
Fake Nationality: Subverted, sort of. The Monarus in the manga are fully European, while the film cast Asian actors in the role, so at first glance this is likely the case. However, it turns out the Monarus of the film are based in Singapore, which is a melting pot of a country, and on top of that, they are at least also partly Japanese, considering that Michelle has a Japanese middle name. This means the film is correct in casting Mariko Shinoda as Michelle. As for her brother, this is a bit more complicated. Nichkhun is an American of Thai Chinese descent, so the Japanese explanation doesn't add up, but since the Chinese are the largest single ethnic group in Singapore, the Monarus might have been also part-Chinese in addition to part-Japanese, so his casting makes sense.
There are also Nekozawa (Japanese of Russian descent) and Renge (European of Japanese descent) respectively played by the very Japanese Ryo Ryusei and Tao Tsuchiya.
Irony as She Is Cast: Haruhi can't sing despite both her Japanese and English voice actresses being good singers. This gets parodied in the Japanese dub in episode 19 when Haruhi apparently sings well, only to reveal she was lip-syncing. The voice she "lip-synced" to however was her real voice.
Real-Life Relative: Hikaru and Kaoru Hitachiin are portrayed by real-life twin brothers Shinpei and Manpei Takagi in the live-action adaptations.
Surpassed the Teacher: Hatori Bisco herself is an example of this, given that she used to be tutored to an extent by Hino Matsuri, the author of Vampire Knight, who at the time was a more experienced mangaka (and Vampire Knight being more popular than Ouran to boot), whom Hatori greatly admired and looked up to. Eventually however, Vampire Knight suffered from Seasonal Rot and a steep loss of popularity along with it, while Ouran continued to climb the ladder of most successful shoujos, making it as far as to the top 10 list in Japan and even surpassing shounens such as Bleach (a rare accomplishment for a shoujo). In contrast, Hino Matsuri was told to end her story earlier than she wanted, her decrease in storytelling-quality and popularity not leaving Vampire Knight substainful enough for the LaLa magazine anymore (same one as Ouran was published in). Ouch.
Apparently, Hatori originally considered Haruhi and Mori to fall for each other. This idea might be referenced in chapter/episode 7 where Haruhi blushes when Mori first calls her by name, and they spend some alone time where she's willingly carried around by him.
The bonus chapter about how Haruhi's parents met was originally going to be a full length chapter, but had to be shortened due to time limit.