i agree with asa. koscielny is a brilliant defender, but he does make more dangerous errors than vermaelen or mertesacker. that stupid pass to silva was an unnecessary risk, and it's the kind of thing that can cost you a match. the fact that city didnt capitalise on it is irrelevant - they could have, and that's the point.
a guy who plays like the lovechild of thuram and maldini for 90 minutes but shanks two clearances for og's is a third division player because he will get you relegated in those 5 seconds of madness. the further back the pitch you go, the less forgivable an error. further forward, more risk-taking is acceptable, even required.