Have you made any bad hires that way? I used to think that I could gauge someone's depth of understanding by asking pointed questions about past projects, but after doing that several times and following it up with a simple coding problem that they totally flailed around on, or having a colleague tell me that they failed a simple coding problem in their interview, I realized that some people are just very good at faking understanding at that level (talking about code but not actually coding). Especially if it's on their "own" project that they're familiar with and have practiced responses to.