This example demonstrates that the 'mousePressedEvent' sometimes is not delivered at once. In that case the 'pressed-event' will be delivered in combination with the following event (maybe mouse-release or mouse-enter or mouse-move..)
 
You would expect to see the 'event pressed' at once after you press the button, which happens only sometimes. In most cases the next mouse-event (i.e. mouse-release) will trigger both events.