The Power of Skills---Right or Wrong
A high school mathematics teacher, telling his student that there was no excuse for his arithmetic errors, said: "You should check your work using a calculator whenever you are in doubt. It's as simple as that."
The student appeared confused, "But," he replied, "I'm never in doubt."
Source: J. Braude's Treasury of Wit and Humor, 1964 (variation of joke #836)