[jarrett]

Very true. Here's an example of a bug that has bitten me more than once, has everything to do with immutability, and isn't reasonably within the scope of traditional testing tools:

A crucial class writes files to a certain folder. The test sandbox provides such a folder, and the class knows to use that folder when it's running in test mode. Obviously, the production environment is also supposed to provide that folder. But oops, it doesn't!

The tests exercised writing a file to disk, the tests were all green, and the system crashed anyway.