Once upon a time I updated a Drupal site and was unable to log in after the update was complete. I tried messing with the $cookie_domain and $base_url to no avail. When I used the Web Inspector or Firebug to view my Cookies I noticed I was not getting a familiar PHP Session Cookie.
Here's how to check:
When working on a local copy of a site that also has a production, stage, and/or development environment on another server, it can be handy to hard-code the "Temporary directory" file path as found on admin/config/media/file-system
Drupal website developers and administrators benefit from its built-in feature to periodically check if any of the enabled modules and themes are out of date. Drupal performs this check daily or weekly, when its cron job is run, automatically. If there are any modules or themes in need of an update, then on the admin area's modules or themes "List" page, you will see the notification:
On some of the Drupal administrative pages, you will encounter a great many checkboxes. This is especially true of the permissions page (User management > Permissions), which starts off with more than six dozen checkboxes for a vanilla Drupal 6 installation. Add a typical suite of contrib modules, and the number of checkboxes increases substantially. As a consequence, you may be forced to manually check or uncheck long sequences of checkboxes, which quickly becomes tedious and time-consuming.
One of the things I really love about Drupal 6 is its "Status Report" page, located at "admin/reports/status" (or "Admin|Reports|Status report" if you're drilling down through the menu system). This is my first stop whenever I think something wonky is going on with a site.