"I'm sorry, sir, there's been some sort of error in the kitchen," is what a restaurant waiter might say to a patron who ordered the fish but was mistakenly served a plateful of worms instead.
Simply put, an error is a mistake. However, that mistake is more likely due to a lapse in judgment or skill than to an accident. A plane crash that occurs after the pilot mistakenly presses the "pilot eject" button is said to be caused by "human error." A baseball play in which the pitcher throws the ball to second base instead of first because he was distracted by a pretty girl in the stands is an error — one that might cost his team the game.