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.
You would think a single shot musket would making "counting" how many rounds you have fired until you needed to reload a "no brainer." The stress of a firefight affects soldiers in different ways. Even having been under fire before, there's no guarantee you will always keep your head. Sometimes, the best you can hope for is your training will elicit an automatic and correct response before your brain has time to register how scary the situation is.