You have a slow speed stall buffet which i presume you are aware of. You also have a high speed stall buffet when in simple terms you are going fast enough that laminar flow over the aerofoil becomes too turbulent. As you increase altitude your slow speed stall increases - ie you stall at a higher speed. If you carry on up eventually your slow speed stall and your high speed stall become coincidental. At or near this point if you fly slower you stall, if you fly faster you stall - so you go home in a coffin.