Chain-fire
Chain-fire, or chainfiring, occurs when the flash from a blackpowder revolver chamber being fired ingnites the charges in surrounding chambers. To say that this can be a startling experience is a gross understatement. It is an unnerving and potentially dangerous occurrence. A chainfiring weapon discharges projectiles when the shooter does not expect and from cylinder chambers which are not in alignment with the barrel.
The use of over-sized balls in loading the revolver lessens the chance of chain firing — to a certain degree. However, to completely seal the chambers, most commonly recommended is the use of a wad or sealing with a thin layer of lube after the ball is loaded.
While some people have suggested (and tried) using materials such as Crisco shortening on the front end of the cylinders to prevent chain fires, this presents one simple problem: it gets pretty runny after a couple of shots or, in hot weather, even sooner. Products such as Wonder Wads, which are pre-lubed work well, as does Hogdon's Spit Ball.