In my first or second year of window cleaning a lady customer messed me about with payment and then gave me a 'not this month, mate', so I dropped her from my round.
A few months later a 'for sale' sign is outside her house and she sees us working and comes running out. With a big smile (unusual) she says, "Can you do me this month?" and I said, "I can't, I'm sorry, I've got to be somewhere shortly, but I will clean your house next month". She was happy with that.
Next month came around and she comes running out - smiling - "You're doing our house today?"
And I sez, "Sorry, mate, not this month!" Her shoulders slumped - she knew she'd been had - and she stropped off.
I love a bit of schadenfreude.