A good example of this is bloodletting, an ancient medical treatment. Bloodletting was a method used to balance the four 'humors'. Blood was one of the four humors. According to ancient belief, all four humors needed to be in balance in order to be healthy. By cutting and letting some blood out, the humors would be balanced.
Many physicians avoided bloodletting, but surgeon-barbers and some clergymen practiced it. Then clergymen were forbidden by a church edict in 1163 and led to barber-surgeons doing it all by themselves! At the time, barbers cut hair, performed minor surgeries pull teeth, and amputate limbs. To this day, some plastic surgeons use the technique.