I have been using my open ID on StackOverflow.com and ServerFault.com without issue for months. Today, I went to log in to SuperUser.com to ask a question - and it won't let me log in. I successfully get redirected to OpenID, I successfully log in there, but when I get redirected back, SuperUser.com which puts me back on the login screen with the error:
Unable to log in with your OpenID provider: The signature verification failed.
I really don't think this is an issue with my openID as I used it to log in here just now, and also tested it successfully on both ServerFault.com and StackOverflow.com in the last 5 minutes. It's only SuperUser.com that has the problem.
I have tested with both FireFox and Safari - same issue.
This bug rates really high on the "will it infuriate users" scale - at least judging by the steam coming out of my ears :) I'd suggest it needs to be fixed urgently.