I used IE to log into Stack Overflow (via OpenID). Then, I installed Firefox. It must have copied the cookies from IE (I didn't pay close attention to the install screens), because when I came to Stack Overflow on Firefox, I was already logged in.
More disturbing, though, was the fact that from Firefox I couldn't log out. I was able to create an account on meta and log in and out from here with no problem, but could not log out of Stack Overflow without cookie intervention.
This bug isn't a big deal...just a paper cut.
Recap - to reproduce (on a Windows machine):
- Clear cookies from and uninstall Firefox if installed
- Log into StackOverflow.com in IE
- Install Firefox, allowing everything to be copied from IE
In Firefox, you should find yourself already logged into StackOverflow.com, and unable to log out (you can click the logout link and then the logout button, but to no avail).
I have followed these steps and successfully reproduced the bug on Windows Vista with IE8. The solution is, of course, to clear StackOverflow.com cookies.