In one screen I have a custom toolbar with a "close" button in it, and a UIScrollView. In the screen that preceded this one I had pushed the current screen into the navigation stack. I had also disabled the navigation toolbar for a larger display area and so the close button would instead be used to call:

- (void)doClose {
    [self.navigationController popViewControllerAnimated:YES];
}

Now here's the problem: the application crashes when the close button is pressed, while the UIScrollView is moving. Here's the debugger stack trace:

0 __forwarding__
1 __forwarding_prep_0___
2 -[UIToolbarButton setHighlighted:]
3 -[UIControl(Static) _unhighlight]
4 __NSFireDelayedPerform
...

and I do have the following line in the debugger console:

*** -[CALayer setOpacity:]: message sent to deallocated instance 0x718d250

(sometimes setOpacity would be setNeedsDisplay or something related)

There won't be any crashes while the scrollview is at rest. What is happening?