No story about chance discoveries is complete without the most famous and fortunate accident of the twentieth century. Biologist Alexander Fleming was heading off on vacation one day in 1928, and was too rushed to put away his work. WHen he came back, he realized he-d accidentally left open some dishes. But his mistake had caused something interesting to happen. There was a moldy growth on some of his dishes, and because of the mold, bacteria didn't grow in those places.