Well, I am all in favor of learning from other's mistakes, but I really don't get all that excited about publicizing my own. Here goes tho, in hopes it helps someone else.
I was out practicing with my 12 ga full choke flinter, when I decided, with a loaded shotgun, to adjust the clay pigeon thrower. The range is designed for modern guns, no slots to rest the barrel of a muzzle loader in. I think that I'm pretty safety conscious, I put the shotgun into a v-shaped corner of a nearby shooting bench. However, I missed by literally a 1/4 inch and the half-cocked lock fired when the hammer brushed past the bench top.
I'm one of those who doesn't believe in firearms 'accidents', it shouldn't have happened, and it did because I was careless. The solution? I've seen this before - A thin and fairly narrow plank with a block of wood on the underside to keep the plank from sliding away from me when put on the top of a shooting bench, a notch in the free edge of the plank, facing me, for the gun to rest in and a shooting box placed on top of the plank to keep it from rising up. Simple and effective. The hole in the roof? Well, I was told by an officer of the club not to worry, its a heavily used police range with constant holes being put in the roof during rapid fire practice. So, I have learned a second lesson - Avoid that range like the plague during rapid fire practice.