We do this to pretty good success at the Big 4 I work at as well. The process by which these post-morterms get assigned can occasionally get political -- e.g. manager A dislikes or is trying to make a move on manager B so he pushes for a detailed post-mortem for every little issue that B's team has -- but the post-mortems themselves tend to work very well in exposing and correcting process failures without placing individual blame.