why would opening http://biology.stackexchange.com/users/openidconfirmhttps://biology.stackexchange.com/users/openidconfirm result in HTTP 404 instead of redirecting me to the main page?
I mean the page is to "confirm the OpenID" and I've already have it confirmed, so it's reasonable to just ignore the attempt
The reason for the 404 is not the fact that you already were logged in. If that were so, you'd certainly have a point.
The reason is that this page is only meant to be requested via a POST request. Since typing the URL into the address bar by hand causes a GET request, you received a 404 (and yeah, I know the really correct response would be a 405).