For most OpenID options, it doesn't much matter if you pick the "Sign up" versus the "Log in" tab on the sign in page. Whether you have an account on the site or not, it just does the right thing. Not so if you are using the Stack Exchange OpenID service:
(Bonus bug: the reCAPTCHA box is a bit too wide for the expanded "Sign up using Stack Exchange" container.)
Since we already have collected an email and password (well, two copies of the password), could we not check to see if they match and automagically log the user into their SE OpenID? If not, the error needs to be reworded somehow to explain that while the email might have been used to create a Stack Exchange OpenID, that doesn't mean they can't log in with that OpenID to create an account. Counterintuitively, they need to use the Log in tab even if they've never had an account there before.
The recovering your account link adds to the confusion since it sends an email that leads off with:
Someone requested an account recovery on The Workplace Stack Exchange for firstname.lastname@example.org, but we don't have an account on this site that matches this email address.
(For people with accounts on other sites and/or Stack Exchange OpenIDs, this paragraph is followed by other instructions that would probably work if followed. By that time I imagine many users are at wit's end.)
To simplify: If a user already has a Stack Exchange OpenID (likely because they have an account on another site) they should be able to use the Sign up tab to sign up for an account or there should be clear instructions on what to do next if they fail.