Obligatory "not today" disclaimer: actually fucked up over two decades ago, but found out today.
At the dawn of theinternet, I made an account on imgur. I figured (correctly) that I wasn't going to use it often enough for the username to matter much, so I just went with the first part of my email adress and a number four times.
Turns out that there is such a thing as nazi number code and it's never a good idea to use an even number of 8s for usernames when you're not a nazi.
TL;DR: Didn't know about nazi code, accidentally selected an imgur username ending with the equivalent of a double Hitler salute
LOL nothing of the sort. I first started using this alias when Quake 2 came out, and I joined a clan called "Crack". I was [Crack]Happy (because I'm such a happy guy) and I just kept using it elsewhere.
Because we were all crack shots. It was a quake competitive team, and headshots from long distances while rocket jumping were a pre-requisite for even being considered.