When I first open up lemmy.world in the browser, I am not logged in, and at top right it says Login. I type my username and password and hit enter. I get the hourglass for a few seconds, then for a split second the word Login is replaced by another word, before displaying my username (wazoobonkerbrain).
I did a screencast (recording) so that I could slow it down enough to see what the other word is and it is “calaei”. The screencast is in webm format and I can share it if you want, I don’t see a way to post it here.
i’ve seen that a few times myself & have seen it reported before as well. I suspect that it’s an undocumented feature.
I think that the other user who pops up is your secret lemmy twin
ah, a doppelganger