In September 1928, Alexander Fleming returned to his cluttered London laboratory after a summer holiday to find, as he often did, a stack of petri dishes he had left unwashed before leaving, containing bacteria cultures he had been studying. Most scientists would have simply cleaned up and discarded the contaminated mess without a second look.
One dish caught his attention: a patch of mold had grown on it, and strangely, the bacteria colony immediately surrounding the mold had been killed off, while bacteria further from the mold continued growing normally. Rather than dismissing it as ruined, sloppy labwork, Fleming isolated the mold and identified it as a strain of Penicillium.
He spent the following months investigating further, discovering that the mold produced a substance capable of killing a range of disease-causing bacteria without harming human cells, a finding he published in 1929, though he lacked the chemistry expertise to purify the substance into a stable, usable drug on his own.
It took another decade, and a separate team of chemists at Oxford led by Howard Florey and Ernst Chain, to turn Fleming's observation into a mass-producible antibiotic. Penicillin went on to save an estimated hundreds of millions of lives, all traced back to a scientist who noticed something unusual in what looked like a ruined experiment.