The client had done up a dummy of his direct-mail card on his PC, using MS Word, and wanted the background of the card printed the same fluorescent green he'd picked in Word.
We explained that (a) the color seen on screen was RGB from a monitor and was virtually impossible to replicate, and (b) the only thing close was a Pantone Fluorescent Green ink.
It was another two weeks before the client finally understood the limitations and available options. In the meantime, he fought us all the way, suggesting that we really didn't know what we were doing, etc., etc.
What was infuriating was that the CEO of our company stood by HIM the whole way... a man who knew data but not printing.