Not necessarily. I think I've fixed my lion's share of other people's code that I have embedded in my own systems. In fact, I've never been up on a Saturday night due to my own bugs. In the afternoon when I'm trying to fix my unit tests sure, but not Saturday at 1am.
In an isolated system, sure. Or extremely well defined fully enforced demarcation points in a large system, sure. However as part of a large business system, that can be completely out of your control.
Especially if there is a demarcation point or highly detailed definition, but multiple incompatible interpretations simultaneously exist.
If the concept of the failure mode doesn't even exist when the unit test is being written, or is in direct opposition to the stated business plan at that time so "it can't happen" then unit tests can't help.
Even worse is political issues. Yeah sure I promised in writing our demarc point is I'd send you an ASCII file but it changed at midnight to UTF-16 strings inside an XML file and you're either going to like it or work elsewhere and I can make this stick. Oh, well, when you put it that way, then I guess that's why I'm working at 1 AM. A contrived example for short simplicity, the real world is much longer and more complicated.
