discourse/discourse — pull request #1577 by akshar100 (2013-10-27 10:03:13+00:00)
Title: Update open_id_authenticator.rb

User.find_by_email(...) may return set of rows and hence it might be a good idea to return the first row else there will be an exception (posted in a separate comment)

Even with the previous code my installation of discourse worked perfectly fine but I ran into problem once I had a multi-site installation. I do not know the underlying details but this change fixed the issue.