I don't know if it occurs too with iOS 6, but in iOS 7 when a mobile that has "sharing internet connection" option enabled, and is connected via usb with my computer, appears a kind of extra status bar that notifies this.
When it happens, the view under navigation bar moves down automatically 20 pixels to fit this notification bar. So far there's no problem, because my view is shown correctly. But in a concrete view, I've because the app navigation two navigation bars, one of them hidden (hidden but still existing and working).
So the system detects that there are two navigation bars, moves down my general view 20 pixels (that's ok to fit the bar), and then moves down the second view that is inside the first one, which has the hidden navigation bar. So at last you see the view 20 pixels under the navigation, separated by 20 pixels of nothing.
Is there any way to avoid this second automatic moving down?