We have recently moved our Wordpress-based website to a clustered environment on Nginx, and since then, WP Super Cache has been serving super-cached pages randomly to logged-in users, even though we have disabled this option in the settings.
I have checked every single possibility to no avail. As soon as I log in, I do appear logged-in, but then when I switch between pages, I randomly get supercached pages (and my little logged in avatar is replaced with the log-in button).
I would very much appreciate your ideas. Thanks.
The problem turned out to be due to the load balancing server in front of our web server having Nginx cache enabled by default (regardless of the login state of users).
WP Super Cache was indeed properly configured and doing its job just fine. Turning off the default Nginx cache on the load balancing server solved our problem.
Thanks to everyone for trying to help.