Fleming went on vacation without washing his petri dishes and came back to penicillin

In August 1928, Alexander Fleming left for a two-week holiday with his bench in its usual state — cluttered, and stacked with unwashed culture plates of Staphylococcus. Returning on September 3, he began discarding them, then stopped at one: a mold colony had contaminated the dish, and around it the bacteria had dissolved into a clear halo. His recorded reaction was not 'Eureka' but 'That's funny'.

The contamination itself was a compound accident — a Penicillium spore that likely drifted up from a mycology lab downstairs, landing on a dish during an unusually cool spell that let mold and bacteria grow in just the right sequence. Fleming isolated the mold, named its active substance penicillin, published in 1929, and was largely ignored: the compound was unstable, hard to produce, and he wasn't the chemist to fix that.

The mistake only became a miracle a decade later, when Howard Florey, Ernst Chain and Norman Heatley industrialized production in time for World War II, and penicillin went on to save hundreds of millions of lives. All three shared the 1945 Nobel Prize with Fleming — though history mostly remembers the man with the messy bench.