The law of probability suggests that this was bound to happen sooner or later, but it doesn’t reduce the shock that I feel seeing/hearing about it.
It looks like the ball got a bit big on him and he was through the shot slightly too early into the bargain. The result is that it hit him under the bottom of the lid and on the reverse side of the head.
The trouble with having a helmet that comes down further behind the ear is that it restricts head movement. I do sympathise with people who say it is daft that there is a gap big enough for the ball to go through over the visor on some helmets, but these are adjustable, and Broad would have been wearing it like that as a personal preference.
The fact is that cricket is a game involving a hard ball and it is a game where it is legal to try to hit someone on the head with that ball (the only game I can think of where this is the case, in fact). There should be no knee-jerk reaction to this.
Very, very unfortunate and I really hope he pulls through.