If someone is partaking in an act of flagrant stupidity (especially if their stupidity might harm someone or something) I feel morally obligated to point out what a fucktard they are. If they're being a fucking moron as opposed to simple 'I didn't know any better' ignorance, I feel totally justified in being a jerk in my reply.
However, some internet communities have a Golden Rule policy. If you can't say something nice or as quoted above, "Be friendly and courteous at all times". While this is a good rule in general...there are some times it just doesn't apply.
Example: Say I'm on a Stupid Chicken Forum which also caters to other animals. Someone posts a long message about how he and his wife (who are totally not novices!) just got a four year old untrained horse and stuck in in their backyard where he drinks out of their above ground pool and likes to play near their windows, and could someone tell him all he needs to know about horses in ten words or less (even though HE IS NOT a novice, thanks), this might get a reply in the vein of: "This was a terrible idea and I wish you luck."
Such a comment is apparently inappropriate under the "be friendly and courteous" clause, and will be deleted.
I'm sorry, but if you're a stupid motherfucker on the internet, the internet is going to be a motherfucker right back. I find rules like that fundamentally retarded in pretty much any community. It's actually insulting. It's as if the admins are saying no, you are not a group of reasonably functional adults who are capable of dealing with disagreements and criticism. You are grade school children and we have to shelter you from these bad, naughty things.
If I ever start my own message board, rule #1 will be "No unjustified dickery. If you can prove a valid reason to be a dick, go right ahead."