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:

An illegal choice has been detected. Please contact the site administrator.

This is Drupal's way of saying "Hey, you tried to submit this form with a <select>, checkbox or radio button option that wasn't included in the original form definition! That's not allowed."

If you go digging for the source of this error message in the Drupal 5 core you'll find it in the _form_validate function. You'll also notice that immediately after the cryptic end-user message is set, Drupal makes a Watchdog entry. Assuming you're logged in with administrator privileges, you can go to Administer > Logs > Recent log entries (admin/logs/watchdog) and see exactly what the offending option was, and which form element it applies to: