The Casimir Effect and Thermodynamic Instability
  One loop field theory calculations of free energies quite often yield violations of the stability conditions associated with the thermodynamic second law. Perhaps the best known example involves the equation of state of black holes. Here, it is pointed out that the Casimir force between two parallel conducting plates also violates a thermodynamic stability condition normally associated with the second law of thermodynamics. 
A. Widom, E. Sassaroli, Y. N. Srivastava and J. Swain