The adhesive for Post-it notes was initially considered a failure.

In 1968, Spencer Silver, a scientist at 3M, developed a new adhesive intended to be super strong. Instead, he created a 'low-tack, reusable, pressure-sensitive' adhesive that wasn't very durable. It was long considered a failure and found no immediate application. Several years later, his colleague Art Fry came up with the idea of using the adhesive to create bookmarks that wouldn't fall out of his hymnal. From this, the Post-it note was born, revolutionizing office work and personal organization worldwide.