As a Java consultant, when they ask me to develop a Java program: no problem.
When they ask me if I can do it in C#: no problem, but then it will take longer because I have to learn the language (read: APIs).

So this exactly illustrates #5: don't expect us to know it if we didn't say we know it.

But of course you're right when you say that you hired a bad consultant if he doesn't know his core-competence.