From The Greatest Mistakes of All Time by Martha Brockenbrough:
Penicillin is another famous example of a mistake 
turned good. In 1928, scientist Alexander Fleming noticed that mold spores had contaminated one of the bacteria samples he had left by an open window. Instead of discarding his ruined experiment, Fleming took a close look and noticed the mold was dissolving the harmful bacteria. And that's how we got penicillin, which helps people around the world recover from infections.
This brings to mind a powerful quote by scientist Louis Pasteur, "Where observation is concerned, chance favors only the prepared mind," and another, by writer James Joyce, "Mistakes are the portals for discovery."
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