[cbd1984]

Changes in the environment screw it up, too. As an obvious case, if your software is screen-scraping a website and the website's layout changes, your software's just rotted even though your code hasn't changed one bit.

Another example is software which interfaces with hardware: Swapping one specific piece of hardware out for "the same" hardware made by a different manufacturer can cause rot, because "the same" hardware isn't necessarily the same in every respect. If your software tickles it the right way, it can expose differences which can cause your software to fail in new and exciting ways. Nothing changed, yet everything's different.

As you expand the universe of things your code has to interact with, this kind of change becomes inevitable. As always, the more points of attachment, the more potential for future pain.