That actually almost happened to me once. Got a job through a temp agency and the last step before getting placed was a drug test. Despite having never done drugs, my test came back positive for meth use. They tried to just let me go, but I raised enough of a stink that they sent me to Occupational Health for a re-test instead. Less than 2 hours later I was re-tested at Occupational Health, and the results came back completely clean. I wound up getting the job after that.
A few months later, I got an apology letter from the temp agency saying they discovered that the location where I was initially tested was reusing parts of their drug test kits without properly cleaning/sterilizing them.