Image courtesy pyxopotamus, used under Creative Commons Attribution-NoDerivs 2.0 Generic License
Our first entry was a mistake and owes its origins to a very serious matter. In 1943, General Electric engineer James Wright was working on developing synthetic rubber. As World War II raged, the Allied countries faced shortages of rubber from natural sources. Wright's experiments resulted in something that wasn't nearly as useful as rubber but would become a popular toy: Silly Putty.
Wright made Silly Putty from silicone oil and boric acid. He dropped some and saw it bounce. Over the next two years, General Electric sent samples of Wright's putty to other engineers but no one could figure out a way to use it in a practical application. By 1949, a toy shop owner got her hands on the stuff and immediately saw that it could be used for entertainment rather than industry.
Silly Putty didn't become immediately popular. A marketing consultant sunk money into it and nearly went bankrupt before a newspaper story in 1950 created a huge demand for the product. It's been available ever since, bouncing on floors and picking up newsprint for decades. Silly Putty falls into the category of serendipitous brilliance -- also known as the happy accident.