You're going to get some posts to this question that say it is possible. Somebody is going to say that they dropped a shell/box of shells and one went off. Maybe they tossed them in the car and ,,, BANG,,, "it just went off". Somebody will point out that a bullet wouldn't shoot out, it would be more like a firecracker if the shell did in fact "go off" without being in a gun.
Yeah, all that is possible, but the odds of it happening, now that's another story. However, the odds are pretty good that the clip will fall out of your pocket and you will loose it. I wouldn't have a reasonable concern about a shell going off in my pocket.
Darn-near impossible. If it happened, you wouldn't be seriously injured, if injured at all. Hatcher did experiments around this. He wrapped various rounds (.45 ACP, .30-06) in cloth, along with a bar of soap (to simulate the body). He fired the rounds electrically. The .45 round did almost no visible damage. The .30-06 put a small dent in the soap. His take was that the .30-06 would cause a nasty bruise.
I agree that the possibilty of centerfire rounds "going off" in a pocket are VERY remote. Encased in a magazine? Almost impossible.
Rim fire rounds MIGHT be more prone, but in more than 40 years of shooting, hunting and fooling around with firearms, I have never had a round fire outside a weapon. Threw a .22 on the fire once ( got jammed in an old rifle and had a severly bent case ). Not very smart, but the only reaction I got was a faint "fizzle".