I've never uttered the word "perfect". Both goals are impossible, but the second one is even impossibl-er, since it's much more global and complex.
Anyway, mistakes've been made and will be made in the future. Will you propose not to incarcerate people at all, because innocents can be sent to prison by mistake? I guess not (if we are talking about real, and not an utopian society). Double standards, eh?