Executive Meddling: Cherry's introduction in the manga series. Roman remarks it was a request from the editor to let there be some fanservice in the series (something that Shinohara often avoided). And oh boy, the main cast know that something is going to go wrong when he appears in it.
First Appearance: With the difference of exclusive characters in both one-shots, and the appearance of the Sket Dan (with minor design differences), Otakura and Yabasawa made their debuts in the first Akamaru Jump one shot, and the rest, they say, it's history.
Meta Casting: At some extents, Momoka, in the anime. Marina Inoue started at a young age (20 years old compared to 17-year-old Momoka) at the seiyuu and singing industry, too.
And Dante, since Gackt was one of his inspirations.
Name's the Same: Chiaki Takahashi, often referred as Captain who shares name with seiyuu Chiaki Takahashi... except it's not Chiaking herself (nor in voice nor shared background).
Some JoJo's Bizarre Adventure one: Kazuyuki Okitsu, Jonathan Joestar, did a minor role in the anime and he has a talk of sorts with Switch. Sugita voiced Joseph Joestar, his grandson in the TV animation series (and the All Star Battle videogame), and is a great fan of Jojo in real life as well (hence his references of the 'WRYYYY' scream in the series could be his inner fanboy as well).
Talking to Himself: Happened plenty in the Sket Dance/Gintama crossovers, especially with Gintoki and Switch. And with Teppei and Shinpachi.
Technically, Bossun's dad never interacted with his son, but his letter made it look like so in the 'Happy Birthday' episodes, and since Hiroyuki Yoshino was both voices (being a bit of a mythology gag from the manga), it could count as talking to himself.
The Other Darrin: As often happens when there are Drama CDs or VOMICs (manga chapters often animated with audio) before an anime series, part of the cast of regulars changed. The only ones to retain their voices were the Sket Dan, and Yuu Kobayashi was often changed to another character not originally voiced by her (from Reiko to Daisy).