Even if you feel strongly about it, think twice before reacting. Having good manners yourself doesn't give you license to correct everyone else. If you do decide to inform someone of a mistake, point it out politely, and preferably by private email rather than in public. 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.
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