Bought my first cap and ball revolver about 1970, when I was 15. It was Italian made: brass-framed 1851 Navy in .44 caliber. Roughly finished but it was a good learner.
On three separate occasions I had multiple discharges with it.
In those days, I was using a .451 ball, DuPont black powder (FFFG), probably Remington caps and Crisco over the seated ball. It was the common advice.
1st Discharge: ball at 2 o'clock (as you're holding the gun away) went off. Missed the frame, from what I could tell.
2nd discharge: Ball beneath the rammer and the same 2 o'clock chamber went off. No damage to rammer noted.
3rd discharge: Ball hit rammer again, and this time the chamber at 10 o'clock went off. Rammer damaged. After that, I junked the gun. I no longer trusted it and it didn't ram properlly besides.
In those days I wasn't pinching the caps into an oblong, to make them cling to the nipples. I now believe that the caps were knocked off by recoil, or fell off just before firing, and the extra chambers were ignited by flame entering at the rear, not around the ball.
I no longer believe that multiple ignition begins at the front, by flame going in around the ball.
Since I now pinch the caps, use .454 or .380 balls, employ a greased felt wad between ball and powder, I have had no multiple discharges. And note too, when I use greased felt wads, I don't put grease over the ball. There is no need.