Over-tightening of wheel lug nuts is the most common cause of warped rotors. Second, is improper resurfacing by cheap machines used in chain auto parts stores. Third, is cooking them through abuse or mountain driving. Buy an inexpensive set of dial calipers and check the thickness yourself, the minimum is usually cast into the hub area of the rotor. If you choose to have them turned, pay a few bucks more and go to a real auto machine shop. When you get them back they should have a cross hatch pattern on both sides, not smooth. Ask about this before you allow them to do the work. Got cash? Get Domestic's slotted new rotors with Bendix ceramic pads. This combination will put you through the windshield!