Casualty activating without sacrifice. Maybe interaction caused by haughty djinn.
People are using casualty without having to sacrifice a creature. I've had multiple decks use Make dissappear for 1 mana without having to sacrifice a creature. It's gotten to the point I think it's being exploited on purpose.
I think this bug occurs when a player cast a spell and selects a creature with an attack value that will change before the ability resolves. In this exact instance haughty djinn can be a 3/4 and you cast make dissappear targeting them to sacrifice. By the time this resolves haughty djinn is a 4/4. guessing because casualty is paid with the attack power of the creature it appears like the spell assumes that haughty djinn was already sacrificed.
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[Deleted User] commented
Maybe read Haughty Djinn and you will understand why the spell had a reduced cost