EDSAC was the first computer built for general computing. One had only to load a stored program and let it operate the computer.
The first three programs worked without error. The fourth, by inventor Maurice Wilkes took many tries to get right. An early version surfaced later and was found to have twenty errors in 126 lines. pdf 
Wilkes wrote what might be the first programming textbook in 1951, with David Wheeler and Stanley Gill. It warns: “Experience has shown that such mistakes are much more difficult to avoid than might be expected. It is, in fact, rare for a program to work correctly the first time it is tried, and often several attempts must be made before all errors are eliminated.”
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In his memoir, Wilkes recalled the exact moment he realized the importance of debugging: “By June 1949, people had begun to realize that it was not so easy to get a program right as had at one time appeared. It was on one of my journeys between the EDSAC room and the punching equipment that the realization came over me with full force that a good part of the remainder of my life was going to be spent in finding errors in my own programs.” wikipedia