The New Yorker has an informative article about adulterated olive oil. If you make soap, getting a mislabeled oil could cause your recipe to be significantly lye heavy or short. Click here to read the article.
An excerpt from the article:
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.)
… fraud remains a major international problem: olive oil is far more valuable than most other vegetable oils, but it is costly and time-consuming to produce—and surprisingly easy to doctor.