Error handling is an important component of error handling that is often ignored at design time. As an example, the Twitter REST API sometimes does not have enough useful information to give back to the user. Worse, some requests that should return JSON return HTML on an error. If you are requesting JSON, the error results should also be in JSON. Bad error handling places an undue burden on client applications and hurt your services. My company has developed a Twitter app which was delayed due to these types of issues.