My favorite is when someone posts 'I'm looking for xx part', to which I reply 'I have one here for you... Gratis', they take me up on it, I go out of my way to not only gift it to the individual, take time out of my day to go to the local PO, send it at my cost and they can't even send a simple 'thanks'...
It's not really the cost of the part, or the shipping expense, or even the lost time out of my day that irks me... But rather the lack of simple manners. Irritating to say the least.
Will it stop me from continuing to do it? Certainly not for friends... But for the guy with 1 post and his only post being a request for parts... Probably not going to happen anymore...