Fantasy Game House Rule #2
Per the standard rules characters cannot shoot into melee combat. On occasion though characters might find a need to or at least want to give it a shot. This was my judgement call at the time and it worked out well enough that we have continued to use it.
Option 1: Shooting without a penalty. The ranged attacker makes an attack roll and then the GM rolls a die to determine which combatant might be hit. Each person in the melee has an equal chance of being struck by the attack.
Option 2: Shooting with a -4 penalty. The ranged attacker suffers a -4 to the attack roll. The GM only checks to see if someone other than the desired target is hit when the attack roll is unsuccessful.
Exceptions and Common Sense
If there is a distinct size difference then common sense should be applied. If two human warriors have charged at a large dragon, their friend the archer should still be able to take the shot with no penalty, the same goes for a group of dwarves fighting a stone giant. On the flipside of this, Should four humans be ganging up on a goblin warlord, there should be almost no chance of an archer safely targeting the goblin involved in the melee.