It's pretty common for folks to create unregistered accounts on Area51: following or committing to a proposal when not logged in prompts for an email address and cheerfully creates a new unregistered account.
On the main site, this adds a "register" link to the top bar:
...that takes you to http://area51.stackexchange.com/users/login:
However, if you end up on the discuss site, the UI is a bit more... Insisting:
All well and good, I suppose - except that those links do not take you to http://area51.stackexchange.com/users/login; instead, they go to https://area51.meta.stackexchange.com/users/signup-unregistered:
But hey, inconsistency is the spice of life, right? Both login forms look like they provide the same functionality, so let's just click "Sign up using Google" and get on with our lives...
Awww...
Well, maybe no one uses Google. Let's try creating an official Stack Exchange login! Enter a password, and away we go...
Yay! Life is looking better. And within seconds, there's an email in my inbox! Surely this is gonna work... Just to click the link to complete registration, and...
Awww.
Recommendation
Redirect https://area51.meta.stackexchange.com/users/signup-unregistered to http://area51.stackexchange.com/users/login.