Prompted by Old Lady Leg's post above, I well remember the day I got a phone call from a theatrical-production-type person saying that a play based upon something I'd written was about to be produced in London.
Great news, you might think - but they wanted to know whether I'd be willing to forego payment because money was tight and, of course, the production would serve to publicise the original work.
I told them that if the play hit the stage without a cheque falling through my letterbox, I'd sue their arses off.
They paid up.