Received via e-mail from a long time client:
"I am not going to be paying your most recent invoice. When I asked you to do [project], you told me it would be 5 days work at £500 a day. You then charged me £2500. Where has the other £2000 come from?"
(Thankfully their accounts department had already paid, so I didn't have to get into higher maths with the CEO...)