Accidentals (notes not indicated in the current key) are common in most styles of music. It's not even mandatory to stick to the 12 notes of the western scale,

Accidentals (notes not indicated in the current key) are common in most styles of music. It's not even mandatory to stick to the 12 notes of the western scale, even in western music. We are attuned to hear notes in twelve-tone equal temperament, but this is only one possible way of subdividing frequency. Musicians playing in the English, Irish, African and American folk traditions often use third, fifth and seventh notes that are substantially flat relative to standard western tuning. Barbershop quartets and other acapella ensembles tend to use just intonation rather than equal temperament. Arabic musicians use a tone system with 24 subdivisions per octave; their scales (maqams) habitually employ the neutral second, an interval that exists halfway between the western major and minor second. Music theory is a useful explanatory framework, but it isn't a wholly accurate representation of musical practice.