I knew I didn't take the narcs but at the same time I was also wondering what would happen if the lab screwed up or something I ate showed up as morphine (yes I know that's unreasonable but it was still nerve wracking!!)
If the people who order the testing think it reasonable to take all those precautions with the person being tested, then it is certainly not unreasonable to worry about what happens to the sample once it is out of your hands and passed along the chain.
In fact, a possible switching of a urine sample in the lab, or along the way is probably the most vulnerable chink in the whole process. That is so obvious. I've personally been in enough laboratories where blood and urine and other testing takes place, and the notion that something might be (unintentionally mind you) misplaced or mislabeled is not far fetched at all. The places are often a sort of low level controlled chaos.
If you take intentionality or nefarious purposes into the equation, I'd say labs are exceptionally vulnerable to manipulation (by insiders). And to simple mistakes as well.
On a personal and non drug-related instance regarding lab mistakes, when back in 1981 I was operated on for a tumor, a quick lab analysis of the tissue while I was being operated on gave results that indicated malignancy of an aggressive kind. Later though, after about a week, more thorough analysis showed it to be a rather benign type. I hadn't been informed of the initial results (luckily - who needs to hear a mistaken verdict of that kind?), but my wife had, since she working in the hospital at that time.
So, just saying, the lab is a big weak spot in the process. With no oversight going on as happens to the person being tested.