What's the darkest accidental discovery someone made?

At the Vienna hospital in the 19th century, something bizarre was happening.

There were two maternity wards. One for doctors and another for midwives.

The mortality rate for the midwives’ ward was 36.2 per 1000 births.

The mortality rate for the doctors’ ward was 98.4 per 1000 births.

It turned out the doctors were delivering babies — after dissecting cadavers and not cleaning themselves.

Moreover — surgeons were washing their hands after surgery on live humans, not before.

Dr. Semmelweis discovered this discrepancy and required the doctors to wash their hands extensively and clean their tools. It led to a drastic improvement in survival rates for women.

This was the beginning of modern germ theory.