My DS2 is always banging his head. It is a little embarrassing when people say, Oh, ANOTHER bruise. Especially because half the time he gets his bumps from his brother being rough with him.
It's a phase, don't worry too much. Tell them you know a mum who let her 20 month old go camping and he tripped over into an open fire and got a third degree burn. It makes you sound better.

(It was me by the way and yep, feeling crap about it still!)