In the aftermath of the Battle of Gettysburg, the Confederate Army was in full retreat, forced to abandon all of its dead and most of its wounded. The Union Army and citizens of Gettysburg had an ugly cleanup task ahead of them. Along with the numerous corpses littered about the battlefield, at least 27,574 rifles (I've also seen 37,574 listed) were recovered. Of the recovered weapons, a staggering 24,000 were found to be loaded, either 87% or 63%, depending on which number you accept for the total number of rifles. Of the loaded rifles, 12,000 were loaded more than once and half of these (6,000 total) had been loaded between three and ten times. One poor guy had reloaded his weapon twenty-three times without firing a single shot. At first glance, this doesn't seem to make any sense whatsoever.
Finally we must consider the high attrition rate of artillery fire, which could engage the enemy at much longer ranges than musket volleys. During Pickett's Charge, the confederate army marched slowly towards the union lines and only began anything resembling a spirited jog once they had closed to a few hundred yards. Throughout the war, both sides were reluctant to fire until they had their intended target within their sights. By the time they reached volley range, cannon fire would have already decimated whole sections of the line, leaving behind dead or dying men clutching fully loaded rifles.
These factors probably all contributed. It's certainly believable in light of other studies that some percentage of soldiers intentionally fired over the head of the enemy, or perhaps double, triple, or quadruple loaded their rifles to avoid firing them at all. But 90 or even 60%? That seems ludicrous. The number of casualties at the battle alone (33,000 between the two sides), not all of which could have been caused by artillery, attests otherwise. Those who misloaded or misfired their weapons were among the lucky ones. Plenty were killed before they could fire off a single pre-loaded shot.