Which of these breakthroughs was developed by accident?

In 1928 Dr. Alexander Fleming returned from a holiday to find mold growing on a Petri dish of Staphylococcus bacteria. He noticed the mold seemed to be preventing the bacteria around it from growing. He soon identified that the mold produced a self-defense chemical that could kill bacteria. He named the substance penicillin.   Fleming published his findings and presented his discovery to the Medical Research Club. To his surprise, his peers showed little interest in his work. Penicillin was labeled a laboratory curiosity and Fleming gave up attempts to purify it. Nearly ten years later in 1937, while investigating microorganisms and the substances they produced, Howard Florey and Ernst Chain uncovered Fleming’s research and assembled a team of scientists to work solely on the 'Penicillin Project'. By 1943, the US had sufficient penicillin stocks to satisfy the demands of the Armed Forces of the United States, as well as their Allies.

Source: Science Museum