Very rarely. I've had a reader corrupt a card before, but that wasn't the card's fault. I use two 512 meg cards, in my camera and I've only lost one or two images ever. Might have been a write issue in the camera, I'm not sure. Only half the image came out, it was weird. I've heard of people leaving CF cards in their pockets and putting them through the washer and they survived. They are very durable.