When adding a second QToolButton to a QDialogButtonBox (which is perfectly legal as a QToolButton is a QAbstractButton) with QDialogButtonBox::addButton(), Qt immediately crashes.
The reason is the following code in QDialogButtonBoxPrivate::layoutButtons(): The code casts prev to a QPushButton. But a QToolButton is NOT a QPushButton, it is a QAbstractButton. Thus the cast makes pushButton=0x0 and thus calling isDefault() immediately crashes the application.

    q->setFocusProxy(nullptr);
    if (!layoutWidgets.isEmpty()) {
        QWidget *prev = layoutWidgets.constLast();
        for (QWidget *here : layoutWidgets) {
            QWidget::setTabOrder(prev, here);
            prev = here;
            if (auto *pushButton = qobject_cast<QPushButton *>(prev); pushButton->isDefault())
                q->setFocusProxy(pushButton);
        }
    }