2017-04-18 : For split cards, the flashback cost you pay is determined by the half you cast

2018-12-07 : If an instant or sorcery card in your graveyard already has flashback, you may use either flashback ability to cast it from your graveyard.

2018-12-07 : If you cast a spell from a graveyard using its flashback ability, you can’t pay other alternative costs (such as that of Foil).

2018-12-07 : If you cast an instant or sorcery with {X} in its mana cost this way, you still choose the value of X as part of casting the spell and pay that cost.

2018-12-07 : You may pay any optional additional costs the spell has, such as kicker costs. You must pay any mandatory additional costs the spell has, such as that of Tormenting Voice.