Note to self: Next time, think twice about brad length when loading up the nail gun to attach the back onto your nice bookcase that you've been building. Yes, the shelves are 9" wide, so the brad length really shouldn't be an issued. But don't forget that the top crosspiece - you know, the one under the top, that is NOT a shelf? That one is only 3/4" thick. So choosing 1-1/4" brads, to attach a 3/8" thick back... well, you do the math.
Yeah, that was a good time to close the shop an go sit on the couch with the kids and watch some TV...
Fortunately, it was easy to fix. I bent over all the brad where they blew through, and planed a piece of stock to 1/8" thick and glued it over top of the entire trim piece.