For anyone who's curious, this oft-repeated expression originates from the novel Shibumi (http://goo.gl/gPuyB, fourth line):
Do not fall into the error of the artisan who boasts of twenty years experience in his craft while in fact he has had only one year of experience — twenty times.
Shibumi was published in 2005. I first saw the same expression in a software development book published in the 1990s. It's probably some ancient proverb that's as old as craftsmanship itself.
Do not fall into the error of the artisan who boasts of twenty years experience in his craft while in fact he has had only one year of experience — twenty times.