This could be a personal issue (i.e. my browser or security doing something), but whenever I visit ServerFault, or StackOverflow, I'm prompted for a username and password in the generic browser message window.
Clicking cancel only leads to it popping up again, and again, and again.
Entering anything at all makes it go away.
If I switch tabs and reload, it goes away.
I'll post more the next time it happens, and do a little more testing to get more info, but have any of you experienced this?