This started when I changed "show last user on sign-in screen" to disabled. This forces one to reenter the user name at each logon, along with a password.
I have two user accounts: Randy1 is a regular user; Randy2 is an admin. After I made the change above, I could still login with Randy1, but Randy2 always gave me an "invalid name or password." At first I thought I was really screwed. No usable account with admin privileges! However, it is weirder than that.
In Randy1, let's say I attempt something requiring admin privileges. I get a popup with Randy2 in it and a prompt to enter my pass