Using an instantiable subclass of QGraphicsItem and uncommenting the commented line generates the following error :
QGraphicsScene::removeItem: item 0x1c6a380's scene (0x0) is different from this scene (0x1ba4c90)

path = QGraphicsPathItem()
text = QGraphicsPathItem(path)
#~ text.setFlag(QGraphicsItem.ItemStacksBehindParent)
window.scene().addItem(path)
for item in window.scene().items():
  window.scene().removeItem(item)