I admit I've never experienced what you describe with a client - at least not to that degree. On the other hand, as another poster stated, with a client you've already established some level of authority and can bring them back down to earth or tap a teammate for help/clarification (unless you are working solo) or give a general response and say you will need to get back to them with more specifics. (Of course, with a prospective client, you are, in fact, interviewing and may not want to do that.)