I don't have to take drug tests for the job where I work, but I have taken them in the past for positions I was applying for. I'm not a drug user. The smell of marijuana makes me sick. Even though I'm not a user, I still get nervous about the tests.
Whenever I have to wait that week for the results I get more nervous. People make mistakes, and someone in the lab where they get the results could make a mistake, and my test might come back positive for marijuana. Test results should be immediate, so you can dispute the results then and there and take a second test if necessary.