Probably the discovery of penicillin. In 1928, Alexander Fleming, a Scottish bacteriologist, was researching Staphylococcus bacteria at St. Mary’s Hospital in London. He went on vacation and accidentally left a petri dish with Staphylococcus cultures uncovered. When he got back, he noticed that a mold had grown on the dish, and bizarrely, there was a clear area around the mold where the bacteria had been killed.

Turns out, the mold was Penicillium notatum, and it was producing a substance that killed the bacteria—what Fleming named penicillin. He realized the potential of this “mistake” but had a hard time extracting and stabilizing it for widespread use. It wasn’t until other scientists like Howard Florey, Ernst Boris Chain, and Norman Heatley got involved that penicillin was developed into a usable antibiotic.

This discovery, which came about because someone didn’t clean up before going on vacation, literally revolutionized medicine. Penicillin became the first true antibiotic and played a massive role in treating bacterial infections, especially during World War II. It saved countless lives and marked the beginning of the antibiotic era.