I see the OP as talking about “integration” style tests (say testing against an api) and not being able to capture all of the OS and similar edge case errors in

I see the OP as talking about “integration” style tests (say testing against an api) and not being able to capture all of the OS and similar edge case errors in the test. I do think this is a hard problem, particularly if you don’t design for it from the outset. It’s a bit of the old classic “reading from the file never returns an ‘access denied’ error, until it does”. There’s ways and means, but let’s not pretend this is simple.