I see a lot of commentary that compares Joe Wilson's 'You Lie!' outburst with the ruckus that often happens in the House of Commons. But one thing you are not allowed to shout in the Commons is that another speaker is a liar. A lot of circumlocutions evolved to bypass this - 'terminological inexactitude' is my favorite (Churchill, of course) - but the ban is for a reason. Once the opposition starts yelling 'You lie!' they have essentially abandoned the deliberative process, by questioning the good faith of a speaker. Without an assumption of good faith or a factual rebuttal, just calling someone a liar abolishes the integrity of the debating process. It ends a conversation. And parliament is about conversation.