I expect that this was just a database hiccup, but the error page suggested I might post what happened here at Meta, so...
From StackOverflow I clicked the footer link to Webmasters (needing to ask a Google Analytics question). A header bar suggested I had been automatically logged in and gave me a link to the FAQ, which I went on to peruse for a quick bronze badge (-:
After reading the FAQ I saw that I wasn't actually logged in, I attempted to create an account with my Google ID and link it to my other StackExchange accounts. The final click led my to a page along the lines of "something bad happened, this is our fault, think about posting it to meta". Meta logged me in immediately, and eventually (IIRC), that error page logged me in to a Pro Webmasters account without any action on my part and redirected itself to the Pro Webmasters homepage.
This is on Google Chrome 15.0.874.102 and OS X 10.7.2,
I realize this is absolutely not a question, so feel free to close it or redirect it to wherever else it ought to have been placed initially.