Why are most inventions or innovations created by accident?

As Peter Drucker has said, “Don’t ignore the unexpected, it’s the richest source of innovation.”

Now this is interesting.

X-Rays - Wilhelm Roentgen

X-rays are a phenomenon of the natural world, and story of their discovery is a fascinating one of incredible chance. In 1895, German physicist Wilhelm Roentgen was performing a routine experiment involving cathode rays, when he noticed that a piece of fluorescent cardboard was lighting up from across the room. A thick screen had been placed between his cathode emitter and the radiated cardboard, proving that particles of light were passing through solid objects. Amazed, Roentgen quickly found that brilliant images could be produced with this incredible radiation — the first of their kind being a skeletal image of his wife's hand.

Pacemaker - Wilson Greatbatch

An assistant professor at the University of Buffalo thought he had ruined his project. Instead of picking a 10,000-ohm resistor out of a box to use on a heart-recording prototype, Wilson Greatbatch took the 1-megaohm variety. The resulting circuit produced a signal that sounded for 1.8 milliseconds, and then paused for a second — a dead ringer for the human heart. Greatbatch realized the precise current could regulate a pulse, overriding the imperfect heartbeat of the ill. Before this point, pacemakers were television-sized, cumbersome things that were temporarily attached to patients from the outside. But now the effect could be achieved with a small circuit, perfect to tuck into someone's chest.

The Microwave - Percy L. Spencer

Percy Spencer, an engineer at Raytheon after his WWI stint in the Navy, was known as an electronics genius. In 1945, Spencer was fiddling with a microwave-emitting magnetron — used in the guts of radar arrays — when he felt a strange sensation in his pants. A sizzling, even. Spencer paused and found that a chocolate bar in his pocket had started to melt. Figuring that the microwave radiation of the magnetron was to blame (or to credit, as it would turn out), Spencer immediately set out to realize the culinary potential at work. The end result was the microwave oven — savior of eager snackers and single dudes worldwide.