There are a other few known problems with proper fixes, and those include:
* not able to be logged in to multiple sites on the same domain at once. The fix for this is: ini_set('session.name', 'examplecom') (note that session name can only contain alphanumeric characters.)
* login works, but you still appear to be logged out. The fix for this is to just press reload.
* Users login to www.example.com but don't show as logged in on example.com. The fix for this is to put in an .htaccess redirect so that www.example.com automatically becomes example.com. See Login doesn't work or must be done twice in the Troubleshooting FAQ.
* You are using an older version of Drupal and PHP 5.2. The fix for this is to update to the latest stable version, which has a fix.
* Using the globalredirect module, which has a known bug regarding logins. Fix is simple: disable it.
