Paul Graham collects some sage advice from the founders of startups he's helped fund.  Preparing for a talk, he sent emails to all the founders and asked them "what surprised them about starting a startup?" According to Paul, asking what surprised them amounted to "asking what I got wrong, because if I'd explained things well enough, nothing should have surprised them."

This is a very powerful question that should be on every lawyers post-matter client survey:

What surprised you the most?

Like Paul, you're asking your clients in a polite way about the things you got wrong (or that they think you did because you didn't communicate well).  And I'm quite certain you'll get powerful, surprising and sometimes harshly critical responses -- which are just the types of feedback you can use to eliminate surprises in the future for you and for your clients.