What are some great scientific discoveries from mistakes and/or accidents?

Question

What are some great scientific discoveries from mistakes and/or accidents?

Answer 1

Try this - the accidental discovery of X rays:

X-Rays

X-rays have become an important tool for medical diagnoses, but their discovery in 1895 by the German physicist Wilhelm Conrad Röntgen had little to do with medical experimentation. Röntgen was studying cathode rays, the phosphorescent stream of electrons used today in everything from televisions to fluorescent light bulbs. One earlier scientist had found that cathode rays can penetrate thin pieces of metal, while another showed that these rays could light up a fluorescent screen placed an inch or two away from a thin aluminum "window" in the glass tube.

Röntgen wanted to determine if he could see cathode rays escaping from a glass tube completely covered with black cardboard. While performing this experiment, Röntgen noticed that a glow appeared in his darkened laboratory several feet away from his cardboard-covered glass tube. At first he thought a tear in the paper sheathing was allowing light from the high-voltage coil inside the cathode-ray tube to escape. But he soon realized he had happened upon something entirely different. Rays of light were passing right through the thick paper and appearing on a fluorescent screen over a yard away.

Röntgen found that this new ray, which had many characteristics different from the cathode ray he had been studying, could penetrate solids and even record the image of a human skeleton on a photographic negative. In 1901, the first year of the Nobel Prize, Röntgen won for his accidental discovery of what he called the "X-ray," which physicians worldwide soon adopted as a standard medical tool.

Plenty more interesting examples in the link.