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

Creating a custom drupal module is little messy for a early age developers. But it is fun to play with the drupal API.
Now a days Im working on a custom drupal 5 registration Form. At first when I started playing with it seems fun. Then suddenly I encounter with a very unknown error message which Reads.
"An illegal choice has been detected. Please contact the site administrator."
debugging this error will take much more time if your Form is big, but there is trick to find the exact error.
to Debug the code Login as admin and go to the Logs and click n recent entry logs there you will find due to which field you are getting the error.