Visualize Backtrace seems to be interesting :
http://drupal.org/project/visualize_backtrace


The drupal system composes a function name based on the form_id variable and the suffix of _validate, then it marks a bit as true in the form object, but does not store the whole function. This saves memory but would be slower than storing the function directly or wrapped in a call context in the hash, we can expect to call it later anyway.

if (!isset($form['#validate'])) {    if (function_exists($form_id .'_validate')) {      $form['#validate'] = array($form_id .'_validate');    }  }

later it does more checks,

    if (isset($elements['#needs_validation'])) 

This shows you that there are more callbacks available.  http://api.drupal.org/api/drupal/includes--form.inc/function/_form_validate/6 .



This is the callback function for the submit,

 $function($form, $form_state);