The "Bevel Brothers" deal with this issue in this month's "Muzzle Blasts" magazine. They conclude (incorrectly I might add) that chain fires only happen from the nipple end. Yes, they can and occasionally do cross-fire when caps fall off or are loose on the nipples. Poorly fitting bullets, both ball and conical, will allow chain fires to occur from the front of the cylinder too. I have seen this several times and the problem was either wrinkled, out-of-round balls, or conicals loaded with a slight tilt. Some replica revolvers also have slightly tapered chambers, where deeply seated balls can move forward under recoil and loose their "seal", making the remaining charges prone to cross-firing. This occurance is difficult to stage as it seems to take a bit of bad luck combined with the right combination of poor loading procedure and componants.
#1 cause of chain fires I have personally witnessed: those short "conical" bullets from the reproduction bullet moulds sold with cap & ball pistol kits---The brass ones with ball and conical cavities. I saw this happen last week at the gun club to which I belong. Checked the shooter's componants, and there they were; slightly wrinkly with obvious casting imperfections to boot!
While chain fires are not usually catastrophic, correct fit on caps, a correctly loaded over-size ball or conical, possibly a felt wad under the bullet and some thick grease on top will totally prevent them from happening.