Then one day I was signed out -- the cookie finally timed out, or was cleared somehow, etc., and I arrived at the login page, and was totally stymied. I always sign up for new services using their_domain@my_domain.tld, and that hadn't been possible here, apparently, because asking for a reminder with stackoverflow@my_domain.tld failed.
Ah, I must have registered with an account at any of these half-dozen sites. Well, I have accounts at all of them -- sometimes multiple accounts for different purposes -- all using different email addresses. I had no recollection at all of which I had chosen; I was totally stymied.
After a few days of trying different ways of getting in, I finally figured out the right one (still don't recall), and then could set stackoverflow@my_domain.tld as my backup email, so now I'm covered... but it was the worst sign-in experience I've ever had.