I'm listening to Gettysburg by Stephen W. Sears and the officers on both sides seem to always exhort their men to "aim low". For instance, General John Gibbon told his 2nd Division infantry:
Do not hurry, men, and fire too fast—let them come up close before you fire, and then aim low, and steadily.—Gettysburg, p. 436
This was on July 3 as Pickett's division approached Cemetery Ridge.
There seem to be a number of variations on the command, such as "fire at their feet", but it always seems to be intended to correct the problem of overshooting rather than undershooting. It makes sense to me that artillery fire could be more effective if the rounds bounded across the ground (and at Gettysburg the Southern gunners tended to fire ineffectually over the heads of the Union line). But I would have assumed the best musket and rifle shots would have been aimed slightly high to account for a ballistic trajectory. This might have been a better question for the late Firearms.SE, but what was it about Civil War weaponry that caused trained soldiers to need reminding to aim low?