What're some of your favorite double-edged mechanisms among biology?
For example, the theory that type 1 diabetes was selected for amongst our ancestors for its cryoprotective side-effects. Or the advantage of being heterozygous for sickle cell anemia, granting protection against malaria. Or iron anemia protecting against the bubonic plague.
P.S. if the topic interests you check out the book Survival of the Sickest. #animals