In the last 10 years I've seen 2 or 3 people say they had a false-positive for Ecstasy (MDMA)...I've always assumed it's from the kavalactone Methysticin.
Most of us don't take drug tests, so we aren't really sure how common false-positives are. I took one out of curiosity once, I had a false-positive for some drug that recovering opiate addicts take...I don't think it was related to kava consumption. I know someone who took the same test and tested positive for PCP, but definitely didn't take PCP.