I've had a number fail on impact -- usually the first time the arrow's loosed. "Bad shaft," I say, "Wish I had known that before I straightened it, nocked it, trimmed it, sealed it, pointed it, and fletched it. Oh, well."
I cull quite a few wooden shafts cause they're bent in more than one direction or I don't like the look of the riffs. Out of every dozen I finish, I'll usually get a couple great shafts and a couple miserable ones.
Although carbons are more hassle to tune, it seems that they're more consistent. But -- and I don't know if it's my mindset, or what -- I seem to make my "first shot" hits more frequently with wood than carbon. Go figure.