Definition of a "Bad Design" in software

I came across this succinct definition of what constitutes a "Bad Design" in software:

1. It is hard to change because every change affects too many other parts of the system. (Rigidity)
2. When you make a change, unexpected parts of the system break. (Fragility)
3. It is hard to reuse in another application because it cannot be disentangled from the current application. (Immobility).

Surprisingly, software having these traits are very difficult or even impossible to be unit tested. Hence unit test is often used to a gauge of these bad traits. Don't believe, try it.