It is the installed base that is the double-edged sword hanging over any successful large-scale software product; both its greatest strength, and its greatest weakness.

In particular, the desire to maintain installed-base compatibility runs contrary to the need to make significant changes.

And even if the organization's software engineering practices and UI designs are excellent, significant changes inherently bring incompatibilities and broken applications.

The bigger and more successful your application gets, the tougher the problem.