Idiots.
A "misfire" occurs when the charge ignites, but the projectile remains in the gun.
What happened is that the Mythbusters "shot out."
Close but no cigar. If the charge ignites the projectile is on it's way, ready or not. If the barrel is obstructed and the projectile is not able to clear the muzzle the barrel will split, swell, or explode to release the pressure from the burning propellant.
A misfire occurs when the source of ignition for the propellant (primer, squib, fuse, or whatever) fails to ignite the propellant and the cannon (or any other firearm for that matter) just sits there. You start to wonder of it's totally safe or if there is a smoldering ember somewhere that will touch it off about the time you start tinkering around with the @#$% thing. Misfires that just "delay" the ignition are often referred to by the survivors of the gun crew as "hang-fires".
The same terminology is applied to explosive demolition charges that fail to detonate on schedule. Remember the "Myth Busters" episode where they had several hundred pounds of TNT suspended from a float in a flooded quarry. They did their trade mark count down three, two, one, pushed the red button and nothing happened. They waited ten minutes and then sent a very nervous JD to wire in another blasting cap. The charge went off w/ the second try. I would have been sweating bullets by then.
Regards,
GtG
PS The good news is you'll never even hear the noise!