Submitting info in a form gives "access denied."

PROBLEM:
I've set up a simple CiviCRM form.
I've been testing it today (as an anonymous user) by entering some info. and submitting, but when I do so it directs me to a page saying:
"Access denied
You are not authorized to access this page."

I checked the access control and have "anonymous user", "registered user" and "admin" checked for CiviCRM.

I was able to submit it as an admin successfully, so I believe it's not a problem with the script in general....

RESPONSE:
Make sure you have this permission: "profile listings and forms" in the Drupal permission page enabled for anonymous users.

Source