In 1928, Alexander Fleming, a British bacteriologist, escaped the London smog to take a family vacation in Suffolk. When he got back to his lab, he discovered he had forgotten to sterilize his petri dishes. They were covered with bacteria. A few even had mold.
As he was cleaning up, Fleming noticed that there was a bacteria-free ring around the mold colonies. He had just discovered penicillin, the makings of an antibiotic that would go on to save millions of lives.
This is one of the most famous examples of serendipity in science — and we're looking for modern examples. They don't have to be as revolutionary as Fleming's dirty dishes. They just need to demonstrate how mistakes, surprises and coincidences can turn into valuable insights in the mind of a curious scientist.