From SKYbrary Wiki
In aviation, coffin corner (or Q corner) refers to the point at which the Flight Envelope boundary defined by a high incidence stall intersects with that defined by the critical Mach number. In other words, coffin corner occurs when, for a specific gross weight and G-force loading, the aircraft has climbed to an altitude where the speed differential between the onset of low speed stall buffet and the onset of high speed Mach buffet approaches zero.
As an aircraft climbs towards the altitude that defines its coffin corner, the margin between stall speed and critical Mach number becomes smaller and smaller until the Flight Envelope boundaries intersect. At this point, any change in speed would result in exceeding one or the other of the limits. In the most critical case, simply turning the aircraft could result in exceeding both limits simultaneously as, in a turn, the inside wing slows down whereas the outside wing increases speed. Likewise, encountering turbulence could result in a "beyond limits" change in airspeed.
In commercial and general aviation operations, flight at altitudes approaching coffin corner is generally avoided. Compliance with the aircraft manufacturer's maximum operating altitude in ISA conditions given in the Aircraft Flight Manual (AFM), if necessary taking account of the difference between actual conditions and those of the ISA, will normally ensure that adequate buffet margins are maintained for both the high and low speed boundaries. In the event that turbulence is encountered or forecast, the cruising altitude should be reduced to ensure that buffet margins are not compromised.