I worked for free
I started my working life as an independent professional (I won't say which discipline) before going to the dark side (airline pilot, never mind I had never intended to become one but that's a different story).
People did not like my fees. They were three to five times what the natives charged and I always made it clear that after I finished things could go your way or against, depending on my findings.
I went to see this client, and as he was signing the cheque (remember those?) prior to my handing over my report, I mentioned the amount of tax he had been overpaying for the last fifteen years and how to claim it back. This wasn't what I had originally been hired for but when I saw this, it only took a little extra to put together the documentation for the guy to make his claim.
Effectively, he earned himself a very nice second pension, apart from paying many times over for my work. After the initial incredulity passed, the guy burst into tears of joy.
Biting the hand that feeds IT
