Clients who can't follow simple instructions, especially the ones which are there to make everything as discreet as possible. Like telling them to park in a particular spot i.e. my parking spot not my neighbours'. But still parking in the wrong place! Then sitting in his car (looking suspicious) because a neighbour was pratting about in her car and he didn't want to draw attention to himself. As though a strnager just sitting in a tiny private car park doesn't look odd at all! I send aerial photos of my road, with my house and parking space marked on them, yet they still can't find my place and park correctly. Jeez! I'm annoyed with them before they even step foot over my threshold, so I then have a harder time gritting my teeth and putting up with their "little ways".
The almost shouting "Hello PT, lovely to meet you" and "had a great time, hopefully see you again some time" when I've specifically asked them to be quiet when arriving and leaving in case we're within earshot of neighbours.
The shared bath or shower where you specifically say it's on the condition they don't get your face or hair wet, then they get clumpsy with the shower head, spraying your head. Or touch your head with their wet hands.