Yerba Mate
mehoket gave me a nice yerba mate set as a going to grad school present. I brought it out today, enjoyed the mate, and felt fantastic afterwards. Then, I went to go do some reading on mate and found that the wikipedia article classifies hot mate as a Group 2A carcinogen. This means it probably causes cancer in humans (if you drink it regularly). The studies are apparently a bit dubious.
Drat.