1. X-Rays
It was in 1895 when one of the most important medical accidental scientific discoveries and invention was made by mistake. Wilhelm Röntgen was examining effects of the passing of electrical current through gasses and realized he had found out the electromagnetic radiation which can go through almost any solid matter. His discovery was immediately recognized as a revolutionary thing in medicine, and it had remained like that ever since.
This wraps it up for our list of accidental scientific discoveries and inventions made by mistake.