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Source: nothing.

According to my study of the historical fiction series Sharpe's Rifles, I concur.

It's kinda a ridiculous series, basically about a British super soldier in the Napoleonic war. However, the author was obsessive about the environment and tactics being historically accurate. It's a fun read if you like historical military fiction.

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.

That's part of the reason they are bunched up like that.

Although, they started in formations like that not so people wouldn't shoot each-other, but so that all the spears were pointed in the right direction.

But, you take the basic spear formation, switch to guns, and you don't have to make too many modifications (especially since the early guns were a lot like spears, they were very long, very solid, and had a bayonet on the end). The line infantry tactics did have people shoulder-to-shoulder, but the lines were only between 2-4 soldiers deep. You were always shooting perpendicular to the line, so even though everybody's all bunched up, nobody except the enemy is on the other side of the barrel.

If everybody's all spread out, the odds of a friendly fire incident are higher. If everybody is all bunched up in straight lines, all you have to worry about is the one or two guys in front of you. And, the end of your barrel goes past their positions anyhow.

https://en.m.wikipedia.org/wiki/List_of_friendly_fire_incidents

It can be an issue. There's a lot of moving pieces on the modern battlefield. You have some general rules to avoid it but there's no guarantee.

Nearly, the technological development that made line infantry tactics essentially obsolete was breach loading, because muzzle loading is rather hard to do in a prone position. Breach loaders already used pre-made ammunition cartridges in most cases, but magazines (especially non-internal ones) were a later development. Nonetheless they already made reloading much easier and also much faster than muzzle loading.

Rifled barrels giving increased range, gun powder that doesn't block vision by creating big clouds, and ammunition cartridges/needle guns played a role as well. But breach loading made the biggest difference to infantry tactics.

Advances in artillery range and accuracy also made line tactics much more dangerous after a certain point. However some more conservative commanders still used line infantry related tactics like bayonet charges until well after the advent of machine guns in the first world war.