Errors:

"Although errors can be disastrous and we do what we can to eliminate them, they are informative. Every action is a probe that bounces off the environment like a sonar signal, returning with the outlines of otherwise unseen causes, objects and processes. Deviations between expected and observed returns tell us that the world is not as we assumed and hence provide an opportunity to correct our assumptions."

Deep Learning, Stellan Ohlsson