Penicillin was discovered because a scientist forgot to clean up before holiday

In 1928, Alexander Fleming left a stack of bacterial cultures uncovered while away, and returned to find mould had killed the bacteria growing around it. He nearly discarded the contaminated dish. That overlooked mould, Penicillium, became the basis of the first antibiotic.

— Alexander Fleming, On the Antibacterial Action of Cultures of a Penicillium — British Journal of Experimental Pathology, 1929