Right back
at the beginning we said we would treat air as an incompressible fluid.
Unfortunately it is not, and at higher Mach numbers we begin to see the effects
of our mistake, like the difference between CAS and EAS followed by the
formation of pressure waves and shockwave. These all depend on Mach number,
although we often cite 300kt as the point at which we first notice things going
wrong it is in fact M0.4