I tried to log in several times just now (via the StackExchange OpenID service), but my login attempt failed. I got an error message like this:
"An error occurred on the server
This event has been recorded.
If you believe you encountered this message in error, please report it."
https://openid.stackexchange.com/error?aspxerrorpath=/openid/provider showing in the address bar, and with "please report it" a link going here)
After retrying multiple times I was eventually able to log in successfully. So, I don't know if this is a transient issue or not, and I don't know if I can reproduce it -- but I thought I'd report it in case it is helpful.