I've never smacked my son, not once, and neither has DH - but at the moment, every time he is either tired, upset or has been asked to stop doing something, he stands and smacks the side of his head a few times and shouts 'uh oh' (his usual exclamation when he knows he's done something a bit naughty/cheeky, or there's something wrong).
It's a bit unnerving to watch, should I be concerned? I've tried ignoring it, laughing, kissing his hand, saying 'no hitting'. The last one really upsets him. He's 16 months.