The frequency is important and so is the answer to "could we have done something different ourselves to make the request work". For example in credit card processing, if the remote network declines, then at first it seems like not your problem. But then it turns out for many BINs there are multiple choices for processing and you could add dynamic routing when one back end starts declining more than normal. Not a 5xx and not a fault in your process, but a chance to make your customer experience better.