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The Clean Permissions pattern is to set up an authorization system that is easy for wanted users to use and understand, while minimizing the amount of trashing and other forms of revertable grief from the unwanted.
Post a clear explanation of the permission model prominently on the wiki. For each action requiring authorization, make sure a clear explanation of the authorization criteria is easily accessible.
The original c2.com wiki has a clear and detailed discussion of its Permission Granted scheme and why it works for them.