"Hello in there! I need you to say who you are, please. BTW, I have a loaded gun in my hand and if you come busting out of the bathroom, you are going to get several holes through you. Since I am NOT an idiot, I am not going to shoot through the door, since there is the real chance that you are my girlfriend. Just give me your name, nice and loud, then sit tight. If you are my girlfriend, no problem! If you are not my girlfriend, I am going to have to ask you to sit tight and wait for the cops to arrive."
Whoops, too late for that, Pistorius. Of course you probably were well aware that it was your girlfriend in there, so this is all academic. Personally, I think that if it was an honest mistake, so what? In that case you blew away your adored lover through sheer stupidity, and as far as I am concerned you still need to spend the rest of your life behind bars, because anyone that stupid cannot be trusted to interact with the human race.