This might be hard to reproduce so I'm going to carefully explain what I'm doing, what I'm expecting to happen, and what happens instead. This whole episode has me concerned.
The whole incident began while I was logged into the Stack Exchange network with Firefox 13.0.1 on Windows 8 CP x64. I had previously authorized an API application to access my Stack Exchange inbox data and I revoked access to it (using this method).
I went to open the application (which isn't really an application - it's a web service) and of course, because the access token was invalid, I was redirected to the Stack Exchange authorization page. I wanted to test something, so I clicked the "log out" link at the top of the page:
So far so good - I was greeted with a page that stated:
"Clicking Log Out will clear all local credentials in your browser."
Sounds good, so I clicked the orange "Log out" button:
This logged me out so I began the process of authorizing the application again - only to discover that I am suddenly logged in again.
What's going on?
Edit: I can reproduce this in Chrome 21.0.1180.15 dev-m.