There are really only two ways to get hit on the head by a parking barrier. One is to stand underneath a raised barrier and purposely allow it to fall on you. That is the easy way, obviously. The other method--and this is where a little diminished mental capacity can go a long way--is to forget the barrier you have just seen rise, step into the space it has vacated and stand with lips pursed while considering your next move, and then be taken completely by surprise as it slams down on your head like a sledgehammer on a spike.