From another forum, sorry we aren't allowed to link to it.
"That's a picture of what's left of my thumb after a cap detonated under it while loading my old 58 Remington years ago."
From an article that appeared in the "Cowboy Chronicle" (a different incident)
"Happens that I am one of those pards who Old Scout mentioned as having a cap detonate under "finger pressure". Yes it can happen! The results ain't fun or pretty! For those who want the gory details, it happened in either June or July (I would have to check with the local Sheriffs office to be sure. It was recorded as a "firearms related injury" at the time and the doctor called in the Deputies to record the details) of 1986 in Marion County, Iowa. I was in a farm lane on my in-laws property doing a bit of target practice when a cap that I was "seating" on my 58 Remington detonated under the pressure of my thumb. Now a percussion cap doesn't have much power, not much more than the explosive force of a cap designed for toy cap guns, but the escaping gases coming back through the nipple hole from the ignition of the powder DO HAVE CONSIDERABLE FORCE!. Without the hammer down over that little hole enough hot gas can escape to do a lot of damage to any digit in its path. When it happened I ended up with a thumb that looked kind of like a peeled banana, only black! The nail bed was laid back down over the knuckle of my thumb, a strip was peeled down the inside radius, and the meaty ball was sort of bulged out. This left the bone exposed on the end of my thumb! (OUCH!) After washing and wire brushing away all the charred stuff the Sawbones was able to pull every thing back together and sew it up by running stitches through the thumbnail and the meaty part. But there was a piece missing that he couldn't do anything about. It really didn't hurt though, in fact I've never felt anything in that thumb again! Apparently the nerve bundle on the inside of the thumb was instantly cauterized. Made fer a bit of a hassle to relearn how cock a revolver!"