  Percolation is a concept widely used in many fields of research and refers to
the propagation of substances through porous media (e.g., coffee filtering), or
the behaviour of complex networks (e.g., spreading of diseases). Percolation
theory asserts that most percolative processes are universal, that is, the
emergent powerlaws only depend on the general, statistical features of the
macroscopic system, but not on specific details of the random realisation. In
contrast, our computer simulations of the leakage through a seal---applying
common assumptions of elasticity, contact mechanics, and fluid dynamics---show
that the critical behaviour (how the flow ceases near the sealing point) solely
depends on the microscopic details of the last constriction. It appears
fundamentally impossible to accurately predict from statistical properties of
the surfaces alone how strongly we have to tighten a water tap to make it stop
dripping and also how it starts dripping once we loosen it again.
