Does Cremate counter flashback, scavange, and their ilk?
If one casts Cremate in response to someone casting a card from a graveyard, Cremate won't be able to target the card just cast. By the time the card in the graveyard is cast, it's no longer in the graveyard; it's on the stack, where it's not a valid target for Cremate.
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. [...]