In Drupal’s Boost module, there is a feature setting a cookie called DRUPAL_UID that then lets Apache (or whatever web server you have) avoid serving cached pages to authenticated users.
Well it seems this feature was broken sometime in 2011-2012 (before July 2012), so you might have banged your head against the wall trying to understand why your login box still appeared after login, or some similar effect.
Well, bang it no longer. One guy (namely jchin1968) did a little debug and found the problem, so updating to boost 7.x-1.x-beta2 should fix the problem. Just download this or any more recent version to fix it on your site… http://drupal.org/project/boost
Thanks rcechang for pointing me in the right direction!