If you code for the “happy” path—and make your pattern matches really strict—you will see it in your error logs every time your application encounter something unexpected. You can then decide on what to do in these discovered cases.
Besides the stuff that is build in to OTP (i.e. Supervision), patterns such as safety-valves and fuses can help you detecting and handling errors in regards to third-party systems—this way you can detect if something is wrong in your circuits and act accordingly 
