Process documentation is an essential part of running a smooth operation. What becomes inevitable along the way is the injection of custom cases. These are scenarios that come unexpected or that stray from the standard setup. These CAN be problematic if you're unprepared. But what's even worse is neglecting to realize that these too require documentation.
While an outlier is simply that - a situation that rises very infrequently... being prepared for them is what helps an operation evolve and grow stronger. Other forms of edge cases will undoubtedly arise. And when they do, a smart process is always prepared.
I've witnessed major failure at all levels simply due to the fact that the process wasn't flexible enough to accommodate new user stories. In the grand scheme of things, it's a silly way to fail - especially since all that's required is a broader brush stroke and a willingness to overcome an operational challenge.