Loading a cap & ball muzzleloading rifle involved nine movements that had to be done in order. Soldiers were drilled & drilled & drilled until it was pure muscle memory. You can imagine that in the stress & confusion of battle errors would occur led to a number of rounds being loaded on top of one another.
Fouling was a real problem. 60% of a black powder charge is not converted into the expanding gas that propels the bullet. The plume of white smoke is the obvious evidence of that. I don't recall off the top of my head the exact number, but every so many rounds was a cleaning round like the one pictured above. It was wrapped in a colored paper. Soldiers believed that the cleaning round was a wasted shot. I never have read of any live fire test of that, but it was what the soldier's believed. Here at Sones River N.B. where I volunteer, the site where soldiers pulling back from the cedar break rallied & refilled their cartridge boxes was identified recently. Hundreds of cleaning rounds were found where the soldiers threw them away. Someone knowledgable on this subject may know if the cleaning round actually worked or not.
The cartridge box held 40 rounds. Of course, soldiers would carry more than that, but at about 40 rounds the bore of their muskets would become so fouled that it became difficult to impossible to load. When firing my .50 caliber flintlock or cap & ball rifles the fouling from black powder makes loading noticeably difficult after about ten rounds. The minnieball was smaller than the bore of the musket to make loading easy & allow for fouling. The cone shaped void in the base of the bullet expanded to engage the lands & grooves.
One of the arts of the infantry commander was shifting units out of the firing line to refill their cartridge boxes & clean the fouling out of their muskets. Plain water sloshed up & down will remove the fouling. It was not uncommon for men to urinate into the barrel when canteens were empty. Boiling water is best, but regardless of temperature, the fouling comes right out.
Firing blanks fouls the bore, but doesn't affect reenacting. One of my friends claims he hasn't cleaned his musket in ten years.
The wall of a rifled musket barrel is surprisingly thin. On sunny days here in Tennessee, the barrel can get too hot to touch just by being stacked in direct sunlight. During extended engagements, the barrel would get hot enough to cause the powder to flash as it was poured into the bore. Sam Watkins, among many others, mentions that alarming phenomenon in his Kennesaw Mountain memoir entry. Others had the ramrod shot through their hand by premature detonations. I have no idea how many rounds you have to fire for flashing to occur & don't know anybody who has tried to find out.
I am not aware of any particular rifle being more or less subject to fouling. Using modern powder solves the problem.
Yes, after actions muskets gathered on battlefields were found with more than one round loaded one atop another. There was no official reporting of this by the armorers who inspected & refurbished dropped muskets. As a result, it is the subject of conjecture that has generated reams of arguments over the years. All that is certain is that an exhausted, frightened soldier could easily load multiple rounds without knowing it. How many times would they do it is anybody's guess; as many as 14 have been reported. As Captaindrew wrote above, it is surprisingly easy to misfire in a group without realizing it. I witnessed what might have been as many as six rounds going off together, that was a memorable blast to say the least.