Yes, shit tends to hit the fan when customers don't know what they want/need. And it happens in almost all customer-driven project since the customer normally only contacts a contractor because they, themselves, lacks the skills to develop the product. Something that almost always means that they also lacks the skills to write the requirements specification. Outsourcing really is hurtfull because of the loss of own competence to even purchase products or development of products.

It is imperative to at least try to have them describe what problem they want to solve, since most customers wants to describe how they want the problem solved. Then, there is at least a chance that missing requirements can be found. Or suggestions about logical extensions they better ask for immediately instead of coming back after having received the first prototype (or after having got the first customer responses back from the end-user market).