Actuarial acrobatics so foul, that they are still talking of it on Feringinar to this day. The Klingons involved thought they had invented a new martial art by way of mathematics, and their deep fiscal wounds would be the stuff of song and wine in Stovokor. Unfortunately, it was a hilariously naked attempt at simple fraud. No double-books, no accumulation of rounding errors, no plausible line-items for non-existent goods, no money laundering, no elaborate fences, no nameless middlemen that aren't middlemen, no real subterfuge. Just plain, conventional, bad math and bogus prices. No, the legend persists not because of how brilliant a scam this was, but rather how something so simple almost toppled one of the greatest houses on Kronos; a practical bankruptcy for a Klingon! That is, until Quark came along and explained the deed in plain, simple, Federation Common tongue (ugh) so that even a baby could understand.
DS9 episode called "The House of Quark", where he accidentally kills a drunk Klingon, is forced to marry the widow. The window's finances are a disaster, so he excitedly goes over all her finances, setting things in order (he enjoyed this part).
See @dejected_warp_core's comment for a more thorough set of events.