Which of these famous scientific products was discovered when a researcher failed to wash his laboratory dishes before going on vacation?

Throughout history, many scientific products and innovations have been discovered by accident or had unexpected origins. These serendipitous moments have led to revolutionary products that changed our understanding of science and transformed industries. This trivia question explores one such famous accidental discovery that became a commercial product success. Test your knowledge of how scientific products sometimes emerge from unexpected places!

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Penicillin (Alexander Fleming noticed mold killing bacteria on unwashed petri dishes)
X-ray imaging (Wilhelm Röntgen left photographic plates near cathode ray tubes)
Post-it Notes (Spencer Silver created an adhesive that wouldn't permanently stick)
Microwave oven (Percy Spencer noticed a chocolate bar melting near radar equipment)