[Development] Usage of Q_OBJECT macro in QObject subclasses that don't use signals, slots, or properties

[Curtis Mitch]

Recently I noticed this sentence in QObject's documentation [1]:

We strongly recommend the use of this macro in all subclasses of QObject regardless of whether or not they actually use signals, slots and properties, since failure to do so may lead certain functions to exhibit strange behavior.

Why is this the case? What strange behaviour could result from not doing so?

[1] http://doc.qt.io/qt-5/qobject.html#Q_OBJECT