Many years ago, near the beginning of my flintlock experience, I inadvertantly started a fire with my flintlock. I had a "Bulls Bag" sandbag, and lined it with a T-shirt to protect the brand new finish on the TC Renegade kit I was so proud of. I took a shot, then walked down range to check my target, leaving the rifle in the sandbag. I turned around to walk back, and observed a fire on my shooting bench. The T-shirt had caught fire, and was in flames. It slightly burnt the stock finish I was trying so hard to protect. Lesson Learned !!
EDIT: the fire was started by the lock and the ejecta from the vent, as the muzzle was well beyond the shirt.