In 1928, Alexander Fleming discovered penicillin by accident, the antibiotic that revolutionized medical care. As a result, untold millions of lives were saved in the decades since from fatal bacterial infections.
Post-it notes
Trying to improve the glue that holds sand to sandpaper, an inventor accidentally discovered a very weak glue that stayed sticky during multiple applications!
Microwave oven
A scientist working on the nuclear bomb noticed his chocolate bar melt in his pocket even though it wasn’t too warm. He realized the microwaves were heating the chocolate and an oven could be invented.