Unfortunately that mistake was all down to Joe, he should have stayed on his line and not come forward at all, I've said this a few times this season he sometimes comes too far out. If, as I have just said on another thread, Joe wants to be an outfield player then he has to give up his goalkeepers shirt!! He should have stayed back and the header would have come directly into his hands.
How Nastasic was supposed to hear a call of keeper, which apparently didn't come until the ball was with him, with all the noise in the background I don't know. As Michael Owen has just said on MOTD2 the screaming of 30,000 fans behind Hart would have drowned out his shout. And, whatever you think of Owen, he has at least been on a pitch with that sort of thing happening which is not a situation any of us have been in.
I still think that Joe Hart is our best goalkeeper and if, as others are suggesting, it is down to new goalkeeping coaches then Joe has to stand up for himself and say "I'm the one on the pitch and I'm doing it my way" and get back to the great goalkeeper he is. (JMHO)