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Describe the Bug:
When you cast Genesis Hydra the nonland permanent that is put in play does not appear to be put in play before the Genesis Hydra. Ex. If you choose to put a Doubling Season in play the Genesis Hydra does not get 2x the +1/+1 counters.

Which card did behave improperly ?
Genesis Hydra

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
Manalink 2015/3/25: Fate Reforged,duel

What exactly should be the correct behavior/interaction ?
Genesis Hydra's ability triggers on casting, i.e when it is put on the stack, so it should resolve and put the nonland permanent onto the bf before it itself enters the bf and thus should be affected by the nonland permanent.

Are any other cards possibly affected by this bug ?
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They're put in play before the hydra is. This is easily confirmable by having an instant in your hand when you cast the hydra.

The problem is that spells that become permanents that enter the battlefield with counters compute that number when they're cast, not when they actually enter the battlefield. This isn't so easy to confirm; you have to be able to play a Doubling Season or similar effect at instant speed. Maybe a Corpsejack Menace with a Teferi, Mage of Zhalfir on the bf.

I'm not going to attempt to fix that in Manalink; there's more legacy interactions than I want to deal with. But I've worked around it in the hydra, so that while it's still looking for these doubling effects far too early, it's at least doing it after it puts the permanent from its own effect on the bf. 7db5c3f.