Reasons unclear

Noted anime directors Seiji Mizushima (Fullmetal Alchemist, Concrete Revolutio, Mobile Suit Gundam 00) and Yutaka "Yamakan" Yamamoto (The Melancholy of Haruhi Suzumiya, Fractale, Wake Up, Girls!) have become involved in a Twitter spat, although the details aren't clear at present. It only became known on December 16, when Mizushima tweeted that he had been blocked by Yamakan. The tweet garnered 2,618 retweets. Yamakan replied that it was intentional and then tweeted — perhaps as an explanation — "Childish emotionalism."

Despite rampant speculation, it is not obvious what started the rift. Mizushima shed some light by tweeting on December 17 that:

If I search myself [online], I get a lot of hits about fighting with Yamakan, but just because I'm blocked doesn't mean I'm fighting with him (laughs). I sort of understand why I was blocked, but I don't feel like we've fought that much until now. Don't spread such meaningless things \(^o^)/

Sources: Seiji Mizushima Twitter and Yutaka Yamamoto Twitter; Images from Area Japan and Naver Matome