In many cases the exact skill set that your project requires may not exactly match available people. That is not necessarily a problem within reason.

A good consultant knows his strengths but also knows how to catch up to speed quickly in parts of the job that lie outside of previous experience. A good consultant is enough of a generalist to know how to pick stuff up on the fly, and has a network of contacts that he can turn to for input.

For some consultant horror stories, browse http://www.worsethanfailure.com