A caster can attempt to use a spell scroll of a spell he can't normally cast (DC of 10 + Spell level) if said spell is on his spell list.
Some subclasses give access, at certain levels, to spells that aren't on the main class's list. For example, the War Domain of the Cleric class gives, at 5th level, access to the normally Paladin-exclusive crusader's mantle spell.
Can a low-level caster attempt to use a spell scroll of a spell that isn't on his class's general spell list, but is on his subclass's added list? For example, Bob the level 1 War Cleric attempting to use a scroll of crusader's mantle.