To be fair, I do think he's a decent keeper, but they all have blips along the way and he has made a number of errors, but he is still young and he'll learn and come back, I'm sure. No need to write him off yet.
Except Hart was about 2 yards outside his area, so could only have headed it at best. I don't know what he thought he was doing coming out like that, it was never his, and he can see the defender.