Penicillin was discovered by a lucky contaminated petri dish

In September 1928, Alexander Fleming returned from holiday to find that one of his Staphylococcus cultures had been contaminated by a mould — Penicillium notatum — and that the bacteria around it were dying. Rather than throwing the dish away, he investigated. It took until 1945 for Howard Florey and Ernst Chain to develop a purified, injectable form, but Fleming's observation launched the antibiotic era and is estimated to have saved over 200 million lives.

Source: Fleming, On the Antibacterial Action of Cultures of a Penicillium, British Journal of Experimental Pathology (1929)