Here's a weird one. Have a content type with about forty fields. When I click Preview on an empty form it gives the error "An illegal choice has been detected. Please contact the site administrator." which you're sure to agree is not terribly useful. I took a look at the watchdog logs and saw that the field throwing the error is a required integer checkbox that had no items selected. So instead of saying "You must select one of the items" it says "sir you are a smeghead" - not very useful, Drupal.