viewDidLoad is called, but viewDidAppear is not, for pushed viewcontroller
<p>I have an app where UIViewControllers are pushed onto a UINavigationController using pushViewController. Once in a while, seemingly randomly, the pushed view controller loads (I know this because viewDidLoad is called), but it doesn't actually appear (viewDidAppear is not called). The app enters a state where the new view controller isn't visible, but presumably is on the screen because I am unable to interact with the view controller behind it. In other words, the UI is completely non responsive, but the app continues to run (background and main thread events continue to fire, such as animations and network calls).</p>

<p>Anyone have any idea what could cause a situation like this? Or any ideas about how to go about debugging the cause?</p>