Sooo. My first post here! This is based on some somewhat crude calculations but should at least give us newbies an idea of 1. what the coffin corner is and 2. what happens if there is a sudden temperature increase.
Note that this graph is NOT made for a specific aircraft, the calculations are based on dummy figures for illustrative purposes. Also, the graph is not valid below FL250 (this is based on a/c being in cruise configuration). There are two flight envelopes in this picture, one at ISA (solid) and one at ISA+30 (dashed). MDD is the drag divergence Mach.
So, if you are at 37 000 ft and have a sudden temperature increase you can find yourself dangerously close to a low speed stall, however adjusting your speed will means you get Mach effects and flutter as soon as you leave the high temp area. You are essentially in a coffin corner situation even though the individual flight envelopes gives you a decent speed margin.