As Mad Scientist said; if you slow down you stall and if you speed up you overspeed (running into buffet etc). The heavier you are and the higher you are the closer the two limits. So if you have just taken off with a large amount of fuel and you climb to your max operating altitude, "coffin corner" could be quite small. I have been in a situation where if I slowed down by 7/8 knots we would be in essence about to enter a stall - and if we sped up by 7/8 knots we would overspeed. I kept a VERY close eye on the speed! After an hour or two we had used a fair amount of fuel and so our weight was lower - now we had (I forget exactly) 14 knots from the stall and 14 knots from overspeed...