So, I am assuming that the 1/2 cock position was : A] a flintlock with the frizzen closed [ other wise no sparks ] or B] a cap lock with a cap on the nipple ? I have heard of rare instances where there is enough residual compound on the nipple or enough force to create a spark even without, but I really don't buy it. I always load with my lock on 1/2 cock , frizzed open or without a cap on the nipple. There are two other issues here. 1st, banging or jarring the gun should not dislodge the tumbler from the 1/2 cock. 2nd....and most important, don't put body parts over the muzzle when loading. An accidental discharge is exactly that. It's why we practice safe gun handling. When I'm on the range, whether acting as Range Officer or not, I call out unsafe practices when ever I see them. Friends or not.