Here at The Watch, we've been keeping a running list of the items that have resulted in false positives on these field tests. That list includes sage, chocolate chip cookies, motor oil, spearmint, Dr. Bronner's Magic Soap, tortilla dough, deodorant, billiards chalk, patchouli, flour, eucalyptus, breath mints, Jolly Ranchers, Krispy Kreme donut glaze, exposure to air and loose-leaf tea. The latter item triggered a SWAT raid on an innocent couple and their two children.
In 2016, a report by Pro Publica and the New York Times found widespread false positives and user errors by cops when administering the most popular tests. Precise error rates are hard to calculate, because some test results can be affected by variables such as weather conditions, user error and lighting conditions. Some studies have shown error rates ranging from 1 in 5 false positives to 1 in 3. But even those disturbing figures can get worse if you create an incentive for a police officer to want a positive result. In 2009, the Marijuana Policy Project used the KN Reagent field test on 42 substances that weren't marijuana. They were able to get false positives on 70 percent of them.