Abstract: The talk presents a case study that illustrates that different boundary conditions can cause remarkably different physical behaviour for otherwise identical quantum mechanical systems. We consider a one dimensional well with Dirichlet boundaries that is separated into two halves by a thin nonpermeable wall. The two half wells are populated by the same number of identical noninteracting particles of the same type, kept at the same temperature. The only difference is the reflecting boundary condition at the inserted separating wall, which is Dirichlet from one side and Neumann from the other side. We calculate the quantum statistical pressure on the two sides and find that their difference - the net force acting on the separating wall - is nonzero and asymptotically diverges with increasing temperature.