Why Does Snell's Law Fail?

Snell's law fails when the light rays are incident normally on the surface of a refracting medium. In this case light passes undeviated from the surface, i.e. no refraction occurs.

What is the limitation of Snell's Law?

Abstract: Because the refracted sound field of a finite beam extremely depends on the size and on the frequency of the generating element, Snell's law is not applicable to calculate the refraction angle of a finite beam.

In which case is Snell's law not valid?

Snell's law is not applicable when angle of incidence is zero as the angle of refraction will also be zero.