On: Akin's Laws of Spacecraft Design (2003)

[hyperpallium]

  15. (Shea's Law) The ability to improve
  a design occurs primarily at the
  interfaces. This is also the prime
  location for screwing it up.

This is the problem with the information hiding approach of ADT, OO and even APIs: makes it easy to change the module internals, but hard to change the module interfaces.

But, TBF, the real problem is dependencies.