It's easy to build up an expert reputation and then depend on it. It's easy to become your group/company expert on such-and-such topic, and then have people defer to you even as your skills deteriorate. What's hard is to actually retain a real level of expertise. You have to continually challenge yourself, question assumptions, and test the limits of what you know.
If you find yourself justifying your answers with "because I'm an expert" on a regular basis, it may be time to reevaluate.