Run the small attached sample. Click on the 1st window to activate it -> crash (see picture of crash point in qnsview.mm)

The debugger says:
"[NSCursor set]: message sent to deallocated instance"
(I have Zombie enabled under XCode 5)

This is a Cocoa issue of Qt 5.x.
After some more analysis of the problem, I found the following:

QCocoaWindow stores a copy of a NSCursor in QCocoaWindow::m_windowCursor (cf. QCocoaWindow::setWindowCursor)
but this becomes a dangling pointer later when:

QCocoaCursor::changeCursor is called, which itself calls convertCursor and the line [cocoaCursor release] is reached

My guess is that this is the root of the problem.