An IUD isn't a "Set it and forget it" device....there's a string that hangs down through the cervix which users are instructed to check after menstruation (when they are frequently expelled by accident) and before engaging in sexual intercourse. No string, no IUD=no protection.
I didn't see where the article specified that it was an IUD, though. It may have been Norplant. Regardless, no birth control method is billed as "100%" reliable. There's the chance that the doctor may have done nothing wrong at all, and this woman was merely that statistical anomaly who ended up conceiving.