That's called chain firing. It did not occur in a properly loaded weapon. It only occurred if the loader was sloppy with the powder or with putting on the caps. [Which did in fact, happen too often.]

Also, those who got mangled hands when this did occur were holding the rifle incorrectly. A Colt was not meant to be held forward of the cylinder. Instead, you were supposed to support your trigger hand with your other hand...like shooting a revolving pistol with a shoulder stock.