Why does God allow the devil to hit us when we least expect it?

The worst things that have happened to me always happen unexpectedly. This is not the best example, but since it happened last night, it's fresh in my brain. I help run a Christian Youth Group. We were playing a game and the ball hit my right hand finger - breaking it (- yes I have oesteoporosis but...it was a ball!!!) It's not as if I'm on top of the world at the moment so that God needed to take me down a peg or two. I guess what I'm asking is why, when our whole aim is to do God's work, does He allow these sorts of problems?