it doesn't. unless the test was wrong or mixed up with another specimen (can happen). while there are occasional false-positive tests for cocaine these tend to be rare. if you have used cocaine in any form (like snorting it or on a cigarette) your test may be positive. repeat the test of course if you know in your heart that you have never had contact with cocaine. you can also ask for two tests - one sent to a different lab.
if you never smoked or injected cocaine how does it show positive in your system when taking a test.