I used this plugin and set it to cache not logged in pages. However when it loaded a page for me it showed me logged in as another user and did so for multiple users. At this point I had to delete the plugin.
Used memcache for all caching. We run on 4 webservers with memcache on every server
http://wordpress.org/extend/plugins/w3-total-cache/
There is not enough information here to understand the issue nor provide advice.
I undesrstood.
He just loaded a cached page which was genereated by another logged-in user.
When you do not have "do not cache pages for logged in users" enabled, it's possible that the cache could contain pages for a logged in user. That is why the setting is enabled by default. :)