One day in 1928, Sir Alexander Fleming returned to his lab in London after a two-week vacation to find that mold had developed on a contaminated staphylococcus culture plate.
The scientist was searching for a “wonder drug” to cure a wide variety of diseases. A moldy Petri dish was not a part of the plan, but Fleming noticed the culture had prevented the growth of staphylococci. Further examination revealed penicillin, a powerful antibiotic that could be used to treat everything from tonsillitis to syphilis.
Sir Alexander Fleming’s careless mistake became one of the most important medical discoveries in history. Thanks to penicillin, the rate of death due to infectious disease is now 5% of what it was at the beginning of the 20th century
Source: http://www.visualcapitalist.com/accidental-inventions/