When observables are composed together, subscription can happen at any time when the program runs. For example, the concat operator subscribes to each observable when the preceding observable completes. In such cases, if the subscription of one observable throws an exception, then the whole program stops. To avoid this, the subscription callback of a custom observable should call the on_error callback of the observer instead of throwing an exception in the case of an error. This allows the subscriber to gracefully handle the error.