I believe you that they mixed up your bottles. That actually happened to my clinic. One month I popped for coke but not opiates. I really didn't do coke. And if I did I sure as the hell would have been on an opiate binge and would have never been negative for opiates but positive for opiates. My friend that same week and her boyfriend in the same clinic got positive for drugs they didn't go either, and they were using the same re-iused urine they used the month before. It turned out the nurse or tech doing urines was incompetent and got fired later that month for getting the barcodes on the wrong samples.
I never had a cocaine positive after that.