It appears that the country of Burundi used a freelance photographer's image on their currency without permission. (If the article goes down, BoingBoing saved a side-by-side comparison for your inspection.) The article adds that
Fajack says he's talked about his case with several lawyers, editors and people experienced in photography and none of them has heard of any other infringement cases involving currency.
The theory must have been that there's no room for footnotes on a banknote. (Though the Bank of England found room for a copyright notice.)
(On a tangential note, Kazakhstan misspelled the word "bank" on their notes.)