This came up in our group. The party tanks, a human paladin and a human fighter, were constantly being pelted from behind, sometimes severely, by their supposed allies.
The paladin solved the problem rather elegantly. In front of the party he swore an oath to his god that he would absolutely slay the next being that struck him from behind with a projectile, if he had to hunt them to ends of the earth.
The party thief died two encounters later from suspiciously longsword-looking wounds. By sheerest coincidence, the paladin favored the longsword. Several other characters deviated their feat-paths to include Point-Blank and Precise Shot.
Problem solved. (And no, the paladin didn't fall from grace, GM ruled that ample warning was provided.)