This is part two in what I believe will be a five-part series of posts on human error. This post is based heavily on Chapter 3 of “To Err is Human” by the Institute of Medicine. The book reviews the current understanding of why medical mistakes happen and its approach is applicable to other high hazard industries as well. A key theme is that legitimate liability and accountability concerns discourage reporting and deeper analysis of errors--which begs the question, "How can we learn from our mistakes?" This post covers why errors happen and distinguishes between active and latent errors.