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Describe the Bug:
AI can pay mana for Necromancy then probably cancel the spell if there isn't any creature in any graveyard but mana still gets spend.Is this correct?

Which card did behave improperly?
Necromancy

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
Manalink 2016/08/27: Eldritch Moon v2, duel

What exactly should be the correct behavior/interaction?
?
I am not sure it is a very complicated card.

Are any other cards possibly affected by this bug?
-

It's legal - under modern wording, the card's just an enchantment until its enter-the-battlefield trigger resolves - but dumb. Animate Dead and Dance of the Dead are worded differently so that there has to be a legal target when they're cast.