Access Denied Backtrace will help you track down the specific place where Drupal is triggering the access denied for a certain role.
Use Case:
Have you ever gone crazy when certain roles cannot access nodes in Drupal? Then you have to start checking permissions on and off just to finish right where you were to begin with? Or even worse get some more permissions control modules so they can override Drupal permissions.
This is a helper module that works along side CCK for Drupal 7. It's designed to allow users to view a live preview of the PHP code they enter into field-value / default-value boxes.