As the aircraft climbs higher and higher the stalling IAS increases- but the VMO/MMO decreases. I beleive "coffin corner" refers to the situation where the difference between MMO and stalling speed is very small- e.g 5 kts or less in some cases. In this situation the aircraft cannot accelerate due to MMO and it can not decelerate due to VS. Remember the barbers pole drops after u pass changeover level because it has to conform to the MMO limit. I believe high flying business jets- Lears, Citation etc are most susceptible to this phenomenon?