Geometric frustration: antiferromagnetic Ising spins on a triangular lattice cannot all be satisfied
In the triangular Ising antiferromagnet, every triangle has 3 bonds and only 2 can be simultaneously satisfied (↑↓). This geometric frustration leads to a massively degenerate ground state (extensive entropy ~0.323 k_B per spin) and no magnetic ordering even at T=0. The system remains disordered for all finite T.