title: How would use of unnamed namespaces in headers cause ODR-violations?
body: <p>In the Google C++ Style Guide, the <a href="http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Namespaces" rel="noreferrer">Namespaces</a> section states that "<em>Use of unnamed namespaces in header files can easily cause violations of the C++ One Definition Rule (ODR).</em>"</p>
  
  <p>I understand why <em>not</em> using unnamed namespaces in an implementation file can cause ODR-violations, but not how use in a header can do so. How can this cause a violation?</p>