Rule 10

Be forgiving of other people's mistakes

If you do decide to inform someone of a mistake, point it out politely, and preferably by private email rather than in public. Give people the benefit of the doubt; assume they just don't know any better. And never be arrogant or self-righteous about it. Just as it's a law of nature that spelling flames always contain spelling errors, notes pointing out Netiquette violations are often examples of poor Netiquette.