As a site admin what do I need to be most careful of when developing on the Timebanks server?

As a site admin what do I need to be most careful of when developing on the Timebanks server?

This is a multi-site environment. Writing bad PHP is the main way to cause harm to other sites on the Timebanks server. Please be selective about any PHP added and have EXTRA caution when doing so.  See the Site-Admin Privileged Access Agreement for more details.