In 1997 and 1998, olive oil was the most adulterated agricultural product in the European Union, prompting the E.U.'s anti-fraud office to establish an olive-oil task force. ("Profits were comparable to cocaine trafficking, with none of the risks," one investigator told me.)
The U.S. might be especially susceptible to such fraud:
The American market, which is worth about one and a half billion dollars, is the largest outside Europe, and is growing at a rate of ten per cent a year. Yet the Food and Drug Administration considers olive-oil fraud a relatively rare problem and does not routinely test oils for adulteration.
Definitely worth a read.