The “coffin corner” is the region of atmospheric flight where the stall speed and critical Mach number of a subsonic fixed-wing aircraft converge near the same value. Near the coffin corner, any increase in airspeed will subject the plane to buffeting, loss of lift, and loss of control due to supersonic air flow over the wings and control surfaces, while reduction in airspeed will cause the wing to stall and lose lift. The U-2 reconnaissance airplane routinely operated near the coffin corner, with as little as 5 knots between stall speed and the onset of Mach buffet. Even if the pilot maintains airspeed within the narrow permissible band, consider the effect of a turn: the outboard portion of the wing inside the turn may stall while simultaneously the outside wing goes supersonic; this is bad.
Two approaches to extreme high-altitude flight are light and slow, such as the Airbus Zephyr solar powered aircraft, which use an enormous wing allowing them to fly very slowly while avoiding stall, and hot and fast, such as the Concorde airliner and SR-71, which simply blow through the transonic region and cruise well above the speed of sound.