I have random drug tests for work and I am use to using the substitution method. I use a friends clean piss and hand warmers. I have done it before numerous times with no problems. Today though the temperature was off. I must have had a bad hand warmer. I remember reading in the past about people who first peed in the cup to get the temperature to read, then dump the dirty urine and fill with the clean. I did that and it actually seemed to work. I did not wipe out the container after dumping the dirty piss however bc there was nothing to do it with. I shook the container the best I could to dry it. Is it possible that if there was any residue left, it could cause me to fail? Anybody have any expierence with this method? I am a heavy smoker btw.