One day, a novice programmer came to a master programmer called Schroedinger  and showed him a program he had just written. He asked Schroedinger, "How can I make sure no bug ever turns up in my program?"

Without a word, Schroedinger took the notebook with the program from the novice's hands and threw it out of the office window.

At this moment the novice programmer became enlightened.

(Look for SchroedinBug and HeisenBug in the JargonFile for more info)