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
My randomly assigned 4 digit discriminator on Discord back when I joined was "1488". Didn't know it was a Nazi calling card until one of the members in the community I was participating in tried to get me banned over it.
Fortunately it only took a month or so with Discord support to get them to change it. I contemplated buying nitro so people wouldn't think I was a nazi lol.