this has happened to me as well. a few times i think.

first off im about 5'6 5'7 180 lbs and by no means is my metabolism fast.

Right the first time I was about 15 and in the emergency room. I had been smoking the whole week before, and at various times in the previous months. They made me take a drug test, and my mom proceeded to say that I "better tell her NOW if I had done anything". So I told her that I had smoked ONCE a few days earlier. Well long story short test came back negative and I wanted to kick myself for not being psychic.

Second time in an emergency room when I was 17. They gave me another drug screening. Some heavier drug that I had used the day before came up, but the marijuana that I had been smoking every weekend beforehand did not.

Possible 3rd time was when I was 18 and hired for one of my past jobs. I was given the information to go take my drug test at a local lab. I had only 48 hrs to do so. I went to the GNC and bought some niacin. I took maybe 3 or 4 tablets and drank a half gallon of cranberry juice. I don't know if that had anything to do with it, but I got the job.

I don't know why these things happen.......but it's fuckin cool ^_^