* AnimeFirst: The anime and manga began releasing almost simultaneously, but the former is the original and the latter the adaptation (anime has a longer lead time than manga). The manga's quite different, and lacks Hajime and Tsugumi except in omakes.
* {{Defictionalization}}: Someone has actually made a real "Hell Correspondence" website. For those who seek vengeance, the site can be found [[http://jigokutsushin.co.nr/ here]].
** [[spoiler: By the way, it doesn't work. Your target will not ''actually'' go to hell, so you need not fear having a certain black mark put on your neck setting ''you'' on the path to hell.]]
* HeyItsThatGuy (the LiveActionAdaptation features several actors from tokusatsu series in both regular and one-time roles. KazukiKato ([[Series/KamenRiderKabuto Daisuke Kazama aka Kamen Rider Drake]]), AyaSugimoto ([[Series/PrettyGuardianSailorMoon Queen Beryl]]), KanjiTsuda ([[Series/KamenRiderRyuki Daisuke Okubo, the boss of ORE Journal]]), MasayaMatsukaze ([[Series/DenjiSentaiMegaranger Shun Namiki aka Mega Blue]]), KazuhikoNishimura ([[Series/ChoujuuSentaiLiveman Jo Ohara aka Yellow Lion]])... the list goes on.
* HeyItsThatVoice: Other than Ai's voice below, a lot of seiyuus make guest appearances occasionally as either Ai's client, or those who got sent to Hell. Including HoukoKuwashima, Creator/KaoriShimizu, Creator/RieTanaka, Creator/JunFukuyama, to name a few. Lastly, MagicKnightRayearth fans will take note that Hikaru's seiyuu Creator/HekiruShiina, after a VERY LONG hiatus, makes a frequent voice work in the third season)
** Not that the main cast other than Ai (see below) doesn't invoke this. MasayaMatsukaze not only does Ren live-action, he does the voicing too. Hone-Onna has the same voice as [[Manga/{{Naruto}} Anko Mitarashi]], while Wanyuudo sounds like [[Manga/{{Bleach}} Zangetsu]].
** Many seiyuus weren't that popular when they played in this series. They got success after that.
** And Creator/NanaMizuki, not typecast for once, as Tsugumi in all three seasons. No, she doesn't get to sing.
** [[Creator/SatomiSato Yuzuki]] is the drummer of the [[Manga/KOn Light Music Club?]] We didn't expect it either.
*** [[Creator/AkiToyosaki Yui]] and [[Creator/MinakoKotobuki Mugi]] appear in the third season, too. The former is even the first client.
** Creator/NoriakiSugiyama gets dragged to hell in season one, but not before delivering some very [[VisualNovel/FateStayNight Shirou-esque]] lines.
** Hey look! [[Manga/SoulEater Maka's]] using the Hell Correspondence!
** Dr Honjo (episode four) is played by Creator/TakehitoKoyasu. [[Manga/SoulEater Excalibur]] gets sent to hell... interesting...
*** The English dub has Honjo played by Creator/VicMignogna.
* ScrewedByTheNetwork: Animax Latinamerica licensed the second season and was supposed to have a dub...and then the channel rebranded.
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