This is what makes me swear like a trooper.
You're driving. You notice someone having a hard time finding room to merge / turn into lane. Put on brakes, make room and wave them through. Then they don't have the manners to put their hand up to say thank-you. I don't know if it's etiquette around the world to put your hand up to say thanks - but it's the done thing downunder.
Yep - that's a sure fire way to piss me right off.