A character model who is part of a larger unit may be lucky enough to avoid death from cannon shot, a boulder from a stone thrower, or a comparable missile, thanks to a warning shouted by a comrade. Possibly one of his companions shoves him out of the way, or pushes him to the ground. A trooper spots the fateful missile heading towards his leader, and shouts "Look out, Sir!" or some such warning, and hopefully alerts the character in time for him to duck or adroitly avoid the hurtling missile. The missile slips past the character and hits the man standing beside or behind him.
If a character is part of a unit and is hit by cannon fire, a stone thrower, or other missiles not governed by the normal target restriction, then roll a D6. On the roll of 6 the character fails to hear the warning and is hit by the missile. Work out damage as normal. On the roll of 1 to 5 the character is alerted to the danger and avoids the missile. The character is not hit and the missile strikes another model instead, so transfer the hit onto an adjacent model in the unit.
Note that some magic spells work like conventional shooting, in which case the normal rules for shooting at characters with arrows, etc, will apply. In other cases magic works in different ways, either like cannons or stone throwers or differently altogether. This is covered in detail in the separate Warhammer Magic supplement together with rules for spellcasting and magic items.