title: Why is self.navigationItem.hidesBackButton not working?
body: <p>I have a <em>UIViewController</em> that is pushed onto a <em>UINavigationController</em> and is currently displayed. When I go to start some asynchronous task inside the view controller, I can set <strong>hidesBackButton</strong> on self.navigationItem to <strong>YES</strong>, and the back button is hidden correctly.</p>
  
  <p>As soon as the task is finished, and I set <strong>hidesBackButton</strong> back to <strong>NO</strong> (on the UI thread, I might add, I've made sure of this), nothing happens. The back button remains hidden.</p>
  
  <p>Has anyone seen this before? What drives me especially crazy is that in my application (the same application), in a different <em>UINavigationController</em> hierarchy, the exact same code works correctly!</p>