"dust" is what remains when an order in the book is *almost* filled, except for a couple of satoshis. The remaining satoshis will later be matched with a new incoming market order.
Such a match occurred today (July 1st) at 20:56:24 UTC for example. 0.00000003 BTC were sold for .0001 USD, which means the trade occurred at 3333 USD/BTC. .0001 USD is the smallest possible USD amount, so it would be impossible to match the 0.00000003 BTC in a different way.