“Program testing can be used to show the presence of bugs, but never to show their absence!”
Edsger W. Dijkstra (1930–2002) Dutch computer scientist
Dijkstra (1970) " Notes On Structured Programming http://www.cs.utexas.edu/users/EWD/ewd02xx/EWD249.PDF" (EWD249), Section 3 ("On The Reliability of Mechanisms"), corollary at the end. <br class="br">1970s <br class="br">Variant: Program testing can be a very effective way to show the presence of bugs, but it is hopelessly inadequate for showing their absence.