Some spellcasters prepare spells, but they can cast certain spells spontaneously as detailed in their class description. A good-aligned cleric or a cleric of a good-aligned deity can spontaneously cast a cure spell in place of a prepared spell of the same level or higher, but not in place of a domain spell. A druid can spontaneously cast a summon nature’s ally spell in place of a prepared spell of the same level or higher. Such spontaneous casting follows special rules for spontaneous spellcasting, such as how metamagic feats interact with casting time, but otherwise functions as normal spellcasting. A multiclass spellcaster can’t cast a spontaneous spell from one class in place of one from another class.
——Rules Compendium, p.139
