Penicillin, microwave ovens and x-ray images are all examples of accidental inventions. The idea for Velcro came about when a man hunting with his dog saw how burrs stuck to the animal's fur. The first evidence of the Big Bang theory was accidentally discovered, and so were chocolate chip cookies.
Penicillin, one of the most life-altering inventions in human history, was discovered by Sir Alexander Fleming after he took a short break from his research in 1928. Fleming was looking for a cure to infectious diseases, but when he left his lab for a vacation, the scientist neglected to completely dispose of his bacteria cultures. Upon Fleming's homecoming, he was shocked to see that one of the cultures had mold growing on it that destroyed the bacteria. After he isolated the fungus and conducted further testing, the world's first antibiotic was distributed.