If you want to see what's going on when Drupal's cron.php script runs, one thing you can do is to set a breakpoint in a module's implementation of the hook_cron() function, and call cron.php directly from your debugger. There you can step through your breakpoints, examine variables, and hopefully zero in on your problem.
It’s easy to take Drupal’s built-in user management features for granted… out of the box, you can configure your site to allow user registrations, reset forgotten passwords, and handle user logins. We had a use case where we wanted to combine the login and registration forms into a single landing page for anonymous users, to encourage users without an account to sign up with a single click rather than hiding the registration form behind another link.
Drupal 5.x does not support HTML image inputs (<image type="input" src="..."/>) out of the box. A common solution to this limitation is to provide your own theming function to handle any buttons you want to turn into images, as described at http://drupal.org/node/144758.
Debugging the Mysterious "An illegal choice has been detected. Please contact the site administrator." Error
The Drupal Form API is a wonderful thing... once you've been using it long enough, it's easy to forget all of the laborious validation and post-submit processing you used to do by hand for each and every HTML form in your solution.
The FAPI does things its own way, though, and if you're building especially complex forms that involve multiple steps or dynamically generated options, you may encounter the following error upon submitting your form: