When a widget has QGraphicsOpacityEffect set to 1.0 and it contains child widgets with other graphic effects, the application first warns about invalid use of QPainter:

QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::translate: Painter not active
QPainter::worldTransform: Painter not active

and soon after the invalid painter is used, which leads to crash in QWidgetEffectSourcePrivate::pixmap().

The problem occurs only when the opacity is ~1.0 i.e. QGraphicsOpacityEffectPrivate::isFullyOpaque is true in /qtbase/src/widgets/effects/qgraphicseffect.cpp. Then in QGraphicsOpacityEffect::draw() the widget is drawn directly without going through a pixmap (see /qtbase/src/widgets/effects/qgraphicseffect.cpp, line 1198). In that case the same paint device (QImage) is used, but the painter is not shared and therefore a new painter is created for the same active paint device.

The attached example reproduces the problem. Running it leads to an immediate crash, but the crash can be prevented by changing the opacity to for example 0.99.