In my experience, I've seen close to 50% of the people that can talk their way through an interview and put up a good game wind up being absolutely useless at actually writing code once you hire them and set them to work. There is a huge disconnect between being able to understand the big-picture hand-wavey ideas of how software fits together and being able to turn that into nuts-and-bolts working code.

Something as simple as having somebody write a little web application that takes a request, calls an external service, parses the response, and returns a transformed response, filters out the bozos wonderfully.