Greetings!
"Friendly Fire" is a very real thing--even on the modern battlefield.
As the English Lord says to King Longshanks in the film Braveheart:
"But Sire, won't our archers hit our own troops in the battle?"
King Longshanks responds:
"Yes, yes, they will. No matter! They're only the Irish!" and then laughs with smug triumph.
So, you see? Even in the case of Adventurers fighting desperately against hordes of monsters...desperate times call for desperate measures! Getting some of that "Friendly Fire" going is just something that comes with the territory.
Any character that *misses* their target, their arrow instead strikes a comrade fighting against the enemy target in melee combat. That's just the breaks. Wizard's "Fire Ball" spells likewise burn the righteous right along with the wicked!
Semper Fidelis,
SHARK