Over the course of today I thought of some more of these.

In the early 1970's a young Austrian body builder, expressed an interest in being in films. He was told that due to his thick accent, strange appearance, and over-muscled form and lack of formal acting skills that he would never make it as a leading man. 20 years and a billon plus dollars in gross box office receipts he averaged 20-25 million a film. That fellow was Arnold Schazenegger.

In the early 60's a new group called "The Beatles" was shopping for its first record deal. The first label that talked to them turned them down because the owner felt that there "was no future in groups with guitars" not only did the Beatles have massive success, but they were followed by The Who, The Stones, Clapton, Hendrix, Led Zepplin, AC/DC and a host of other guitar centered rock.

When handed the specifications of a certain animal, weight, wingspan, wing shape, aerodynamic drag etc, a group of aero-engineers concluded that this paticular animal could not possibly fly.
The animal you ask: The comon Bumblebee

In 1980 President Ronald Reagan asserted that the Soviet Union was a paper tiger, economically failing and ready to fall if given the right push. He was promptly denounced and ridiculed by every foreign policy expert and talking head on TV as naive. He persisted with his secret plan to bankrupt the USSR. In 1991 The Soviet economy imploded and a new government arose.