  Within the Landau-like approach we study anomalies of elastic moduli at phase
transitions in thin films on substrates. We consider the case where, similar to
many experimental cases, the first-order transition in free crystal would
convert into a second order in the film if the system remained homogeneous. It
is shown, however, that apart from its questionable thermodynamic
profitability, the homogeneous state of low-symmetry phase may become
absolutely unstable which is signaled by changing of sign of its bulk modulus.
