  We study the edge and surface theories of topological insulators from the
perspective of anomalies and identify a novel Z2-anomaly associated with charge
conservation. The anomaly is manifested through a 2-point correlation function
involving creation and annihilation operators on two decoupled boundaries.
Although charge conservation on each boundary requires this quantity to vanish,
we find that it diverges. A corollary result is that under an insertion of a
flux quantum the ground state evolves to an exactly orthogonal state
independent of the rate at which the flux is inserted. The anomaly persists in
the presence of disorder and imposes sharp restrictions on possible low energy
theories. Being formulated in a many-body, field theoretical language, the
anomaly allows to test the robustness of topological insulators to interactions
in a concise way.
