I have seen Stack Exchange cross-site signup: login with Stack Exchange doesn't work and noted that it is marked as "completed", but that doesn’t square with the experience I just had.
Just today I tried to log in to Area 51. I had never used it before. Clicking "Login" redirected me to Meta and then back to Area 51, where I only got an unhelpful message 'User not found'. I thought maybe there was a cross-site privacy/cookie issue at the root, because I was clearly logged in at e.g. meta.stackexchange.com.
Only out of a failure to see any other way to proceed did I try clicking 'sign up' (intending to do whatever I need to in order to sign up), which instantly created my Area 51 account with my SE credentials.
Now if it’s that easy, why don’t I have a one-click, well-explained option to do that when I try to 'log in'?
And why is the previously linked Meta question marked as 'completed' when clearly it’s not, in this case?
Update now that I understand why Area 51 is special here: The UX could be massively improved simply by adding content to the existing error message. Something like, “If you already have a Stack Exchange account but have not used Area 51 before, just click ‘ Sign up.’” Since no logic needs to change, that would be a very low-risk way of addressing this.