  Metric perturbations in General Relativity are usually separated into three
distinct classes: scalar, vector, and tensor. In many cases these modes are
separable, i.e. they satisfy independent equations of motion for each mode.
However, in the present paper we argue that in many cases tensor and scalar
modes are not separable, no matter what gauge conditions are chosen. The
propagation of any of these mode depends on the other. A realistic example
providing such mixing is presented.
