The message was: You are now logged in as !bzt. That was strange, and I just thought, "probably a bug, let's go to the main page. I was suddenly logged in as Elian, and I had all rights which elian had. I logged out immediately
If it happens to you or anyone else again, please do not log out.
Save a backup copy of the cookie file/details, jump onto IRC and do a /join #wikimedia-tech , and tell them that you're logged in as someone else when you shouldn't be.
I'm honestly not sure what'll happen then, but as far as I'm aware we have yet to capture this whilst it's happening (rather than after-the-fact).
All the best, Nick.