The liquid nitrogen would boil when it hit the liquid water. The keg would overflow. I doubt very much you'd be able to seal the keg while it's overflowing. If you did seal the keg, you'd have to hope it doesn't explode since you have no way of knowing how much of that liquid nitrogen remained in there and hence how much pressure it would create. Pure liquid nitrogen at room temperature exerts a pressure of ~2900 atmospheres. The blowoff valve on the keg will vent pressure, but probably not nearly as fast as the liquid nitrogen is evaporating, and hence the pressure in the keg will likely rise and could very well cause an explosion.