In accordance with the famous Uncertainty Principle, Heisenberg is only probably the best lager in the Discworld, or even in Überwald. The Heisenberg Uncertainty Principle also applies when you have drunk enough lager. After quaffing fourteen pints you can know where you are, or what direction you are staggering in, but not both at once.
Sold at Swear and Wells in their famous Food Hall.
Doubly, to Carlsberg beer and Heisenberg's Uncertainty Principle in quantum physics. (The position and momentum of a particle cannot be simultaneously measured with arbitrarily high precision. - ie, you can know where it is or what direction it's going in, but never both at once.).