API docs will tell you how to use an API. But when things go wrong, you can be left on your own. Error messages are often incomplete, misleading, or unhelpful. And what are you supposed to do with NoMethodError: undefined method '[]' for nil:NilClass, anyway? When you learn an API, framework, or library, you can’t just learn how to use it when things go well. You also need to figure out what to do when it hands an error back.