So the story goes that user jessemoforice had forgot his log in info a few years ago and got “very angry” at Mojang for this. When he created his new account he decided to name it Mojangsucksd**k and well Mojang didn’t like it too much. Instead of just banning him they decided to make an adjustment.
Mojang support staff Marc_IRL states that they routinely search their server handles due to harassment on players. Think “username”sucksd**k being very a common form of harrassment since Mojang introduced the ability to change usernames.
“I was actually looking for %sucksdick, due to an unrelated report of harassment. Usually we let servers handle all moderation but it was a parent and a kid, and well, I was just covering some bases. I never did find the username they reported (they couldn’t remember the exact spelling), but I did find this gem. Glad OP was a good sport about it.” – Marc_IRL. Mojang Support
Source: Reddit
