Patching is important. VERY important. But quite simply, software is complex. And sometimes, even testing doesn't catch everything. This is why *I* don't apply Microsoft patches for a minimum of 10 days (second Friday after patch Tuesday):
https://blogs.msdn.microsoft.com/dotnet/2018/07/20/advisory-on-july-2018-net-framework-updates/
That said, if you have the resources to test your own systems, it's another reason you should be testing patches. Using a subset of systems, test group, etc.