> 20. Wherever there is modularity there is the potential for misunderstanding: Hiding information implies a need to check communication.

This is a really great point. Along the same vein: modularity has a cost too. A cost in terms of communication. The benefits of modularity should be weighed against this cost.

We're really good at talking about new patterns to increase modularity, in particular in the OOP tradition (always hide data, every data structure is its own little module with a public interface). We're very bad at talking about the costs associated with these.