Olive oil defects typically arise from improper handling during harvest, production or storage. The most common defect is "rancido" (rancidity), caused by oxidation.
"Riscaldo" occurs when olives are kept in piles too long before pressing, causing fermentation. "Muffa" is a musty smell coming from moldy olives.
Recognizing defective oil is an important skill. If olive oil tastes greasy, waxy or reminiscent of rubber, it has a defect. Quality extra virgin olive oil is always fresh and lively.