There was a post on Reddit a few days back about how to bypass it when people do this. I haven't had it happen to me, but if you have a creature in play (like your Cairne) you can act like you are going to attack with it creating your own arrow (and removing his), cancel, then play any spell that you want from your hand.
A video demonstration of how to beat the targeting arrow exploit : hearthstone
A video demonstration of how to beat the targeting arrow exploit : hearthstone