No, it is not common. First, always presume that loaded gun is going to fire while you remove the slow match (matchcord) from the serpentine (cock). You must control the gun, muzzle and all. It could go of when you are installing your match, "trying" your match, opening the pan, be a delayed fire once you have dropped the match onto the pan, etc. You are responsible for the point-of-impact and recoil too.
Note that poorly home-made matchcord is like the American breakfast cereal Rice Krispies -- it "snaps, crackles, and pops." There are crystals of potassium nitrate on the outside of the cord which ignite and fly off as sparklers which could detonate black powder. "Too strong too long" a solution soaking. Wipe the outside down with a damp towel and redry...
This could happen again however unlikely -- don't be surprised, be prepared. You'll be fine!
I had a similar late ignition that surprised the hell out of me firing a demo at one of our military academies -- I had already un-vised the slow match and was literally sticking it into the pan by hand. It just was not going off! Finally, it went off a couple of seconds AFTER I had last pulled the match out of the pan. My face was off the stock so that I could "blow off my coal" and see the pan, but I never removed the musket from my shoulder, always kept the muzzle level downrange, and the vent hole stayed out in front of me.
Happens to the best of us. And me too.
Regards;
Alden
PS: might wanna consider lightening that powder charge up just a bit...