Can the Earth's crust be in a state of isostasy?

It is now assumed that the earth's crust tends to reach isostatic equilibrium if there are no external forces acting on the crust. This statement is one of the main principles of geophysics, yet it is incorrect. The isostatic state is generally unstable and differs from the stable position of the crust, which actually exists in many regions. These positions coincide only where the crustal and lithospheric thicknesses are constant. They differ considerably when these thicknesses vary strongly in a horizontal direction. Thus many regional isostatic anomalies in orogenic regions are explained. The instability of isostatic equilibrium is associated with the asymmetry of the upper and lower lithospheric boundaries and with large nonhydrostatic stresses inherent in crustal thickness inhomogeneities.