In 1968, scientist Spencer Silver at the 3M company created a very weak adhesive that would peel off when removed from any surface. The funny thing was that he was in fact trying to create a super strong adhesive No one thought there was any use for such a product, until another scientist, Art Fry, realized that the little pieces of paper made great bookmarks for his church songs without leaving residue on the page.
Or take the invention of potato chips:
In 1853, in a New York restaurant, when a customer complained that the fried potatoes were too soggy and thick, and repeatedly sent them back to the waiter, the chef- George Crum- got so fed up, that he took the request for a thinner potato quite literally- he cut the potatoes into thin slices, fried them, and covered them in salt. And, Voila! The most favorite snack in the world was born! [source for both: www.storypick.com].