Alexander Fleming discovered penicillin in 1928 because he forgot to clean up

Returning from holiday on 3 September 1928, bacteriologist Alexander Fleming found that a stray mould had contaminated a petri dish of Staphylococcus bacteria, and that a clear ring around the mould showed something in it was killing the bacteria nearby. He identified it as Penicillium notatum, but the accidental find sat mostly ignored for a decade until Howard Florey and Ernst Chain turned it into a usable drug.

— Standard historical accounts of Alexander Fleming's 1928 discovery, Discovery of penicillin