[Prior] to 1954, experts believed that running a mile in less than four minutes was beyond human capability. People assumed it was an insurmountable human limitation--not possible. Some believed that even if you could, your heart would explode. But in 1954, Bannister broke the four-minute-impossible-barrier and clicked in at 3:59.4.
If--prior to Bannister's run--some of them had missed the memo on the whole heart-exploding thing, chances are the record would have fallen sooner.
Creating Passionate Users: The Clueless Manifesto