Some ranged attacks, such as stone throwers and cannons, or the breath weapons of Dragons, for example, have unique targeting rules, which allow the player to deliberately aim his shot at a character inside a unit.
In the case of missile weapons that can be aimed in this way, there is a special "Look out, Sir!" rule that allows characters to avoid destruction thanks to a warning shouted by a comrade. This is intended to prevent characters becoming victims to these weapons in a manner that is definitely unheroic!
This rule applies to all war machines that work in a different way to ordinary shooting (ie, they don't fire against the unit as a whole, rolling to hit using their Ballistic Skill), and can be aimed at characters who have joined a unit. It also applies to any other ranged attack that uses a template (like some spells and magic items).
If a character is part of a unit consisting of five or more rank-and-file models and is hit by such ranged attacks, roll a D6. On the roll of a 1, the character fails to hear the warning and is hit. Work out damage as normal.
On a roll of 2 to 6, the character is alerted to the danger and avoids the missile. The character is not hit and the missile strikes another model - transfer the hit onto any rank-and-file model in the unit.
If a character model (including his mount) has a unit strength of 5 or more, then he cannot benefit from the "Look out, Sir!' rule.