Yes, it happened to the chap standing next to me on our range way back around 1998. Set off the two next chambers (not one either side, two on one side). No one was hurt and no apparent damage to the gun. He only stopped because the cylinder was jammed so he couldn't cock it. We were all beginner muzzleloaders then. He was using Pyrodex, no filler and no wad. We put it down to no wad. He continued shooting with wads like the rest of us and none of us have had a repeat experience since. Now I have heard various theories regarding is it ignition via the chamber mouth or the improbable loss of a cap followed by ingress of blowback through the fired nipple. Can't think of a way to determine which is more likely, or are BOTH possible? Dunno other than to point out that using wads seemed to correct the problem - or just lucky.
True freedom includes the freedom to make mistakes or do foolish things and bear the consequences.
TANSTAAFL