"A sensor error in a self-defense situation could prove fatal."

I've read stories where a grip safety(a button that is pressed down when you're gripping the handgun properly) has slowed down the person just enough to let the bad guy shoot him a couple of times before he could return fire. Safety systems probably save more innocent lives than the they've hurt, but I bet anything less than perfect in a smart gun is really going to hurt popularity among gun enthusiasts.