I would actually generalize it to:
Don't hack core or contributed modules/themes.... it causes all kinds of upgrade problems.
-----Original Message----- From: support-bounces@drupal.org [mailto:support-bounces@drupal.org] On Behalf Of steeph Sent: Thursday, September 20, 2012 8:01 AM To: support@drupal.org Subject: Re: [support] Ten Commandments
What about the #1 rule: Don't hack core.
Or is it too obvious to include it? ;)