I had a good one a few months back.
I do this detached 30's house in a nice part of Bristol and the couple are really nice, self made shop owners ex council estate and treat everybody respectfully. Often they aren't in but they hide the key and let me use it.
Anyhoo I'm scrubbing away and this woman and her friend (in their forties) come jogging along (she's a bit of a BobFoc) and she haughtily says:-
"What do you think you're doing?" In a demanding sort of voice.
Me: (instantly deciding that as she hasn't asked me in a nice way I'm going to be awkward but truthful): "Why do you ask?"
Her: (Right old strop on) "This is my brother's house and I'm staying here and how did you get into the back garden?"
Me: "I opened the gate and walked in."
Her: "Does he know you're here?"
Me: "I doubt it."
Her: "Well I'm not paying you!"
Me: "I didn't ask you to."
Her: "You're very rude!"
Me: " You started it! I'm here minding my own business, getting on with my work, letting myself in with the key your brother has provided and you come along all demanding - you only had to say "excuse me do you mind me asking, this is my brother's house ..." and I would have been disarmed by your charm and given you a ready explanation."
Her: "Humph!"