When moving the mouse over widgets, then a leaveEvent() is not always sent for widgets. This occurs when a mouse moves into a widget which causes it to resize slightly (to get a zoom effect) then when moving the mouse out of the widget it does not get the leaveEvent(). This doesn't happen all the time, but it is consistently reproducing fairly often.

The attached example shows this happening when you move the mouse around the squares, you shouldn't see a trail of white squares as they go back to black once the leave event is received.