What made it unprecedented, wasn't that Wilson called Obama a liar. It is that he did it during an State of the Union Address. Presidents have been called liars before, but not in that context and not as an interruption to a SOTU Address. One has to recall that from Jefferson until Woodrow Wilson, the Constitutionally required report by the President to Congress was a written document, so there was no opportunity for any kind of unruly behavior. When Woodrow Wilson chose to address Congress in person, the Congress complained that it was too much like a "speech from the throne." And it wasn't until Franklin that the name we know this address (The State of the Union) was actually applied. Although, the address has at times brought out cheers or boos, it has never been interrupted as Joe Wilson did. Please recall that the opposition has always since Roosevelt's time, followed the address with a rebuttal, so they had ample opportunity to reply to a President's address.