Serious question from a rather naive non-DBA:

Say your organization is running very complex software and a database licensed from a third-party vendor, and it contains thousands of tables in regular use. Is there really a way to test every permutation of things that might go wrong? I've looked on with a mixture of worry and pity at DBAs given a job like this to do. And whose responsibility is it to ensure the CUs stay current? The vendor or our DBAs? It's my understanding that some of our vendors have historically adopted the attitude that "it's our system, we've tested it to work on release X build Y (which might be ten years old) and we advise you NOT to tinker with that setup. If you apply a CU and it breaks our system, you're on your own."

Is this a common issue?