[chakspak]

I generally agree with you, though I want to comment on this line:

> Code that you don't touch doesn't suddenly change its behaviour one day.

This can and does happen all the time, when the platforms and abstractions your code builds on change underneath you. This is why a compelling environment / dependency management story is so important. "Code rot" is real. =P

[globular-toast]

I thought about this, but technically that is still the code changing, it just happens to be in your dependencies rather than your codebase. The only reason you really have to change dependency versions is security fixes, and they should be infrequent enough that you could do manual testing. So I don't think it's a compelling reason to write unit tests, although it is certainly an added value.