the first shot, when it was still a mistake, was the best
The art is in repeating the same mistake
I should think he meant the serendipitous (and I now know where that word came from, thanks to Anu's book) kind of mistake, such as penicillin and Teflon. For more on Teflon:
http://www.whitetrout.net/chuck/Teflon/teflon.htm