A sub for Historymemes

From the same unit? Not very large to my knowledge. Everyone is setting up and firing at the same time, and at the same target(s), so there's less of a 'panic moment' (at least insofar as aiming and firing is concerned), and you're close enough that your musket would be hard to get into position to shoot the guy directly in front of you.

From other units? MASSIVE. In Gettysburg in the US Civil War (admittedly a bit after the Napoleonic-era Brits there) there were numerous incidents in which soldiers from the same side fired on each other because of uniform misidentification.