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Describe the Bug:
A) If there's an Enchanted Evening and a Hidden Gibbons on the bf, and the Gibbons gets triggered, it's an enchantment creature - Ape.

B) If you trigger your opponent's Hidden Gibbons by casting an instant, turn it back into an enchantment with Enchanted Evening, and cast another instant, nothing happens.

Which card did behave improperly ?
Hidden Gibbons.

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
Dev, shortly past EMN.

What exactly should be the correct behavior/interaction ?
A) It should lose all its current types and subtypes (not supertypes) and gain Creature and Ape. That is, it shouldn't still be an enchantment.

B) It should trigger again, again losing all its current types and subtypes, gain Creature and Ape, and get its base power and toughness reset to 4.

Are any other cards possibly affected by this bug ?
Everything templated like it. They're mostly (all?) in the Urza block. Incomplete list: Hidden Ancients, Hidden Guerrillas, Hidden Herd, Hidden Predators, Hidden Spider, Hidden Stag, Lurking Skirge, Opal Acrolith, Opal Archangel, Opal Avenger, Opal Champion, Opal Gargoyle, Opal Titan, Veil of Birds, Veiled Sentry. For A only, also Darksteel Mutation and Song of the Dryads.

Fixing B before A is a poor idea, what with the loop it'll cause with Hidden Predators (at least if this is fixed too) and Opal Avenger.

I'm not really sure about A... These kind of interaction are really complicated, there's a ruling stating that the Ape shouldn't be an Echantment, even with Enchanted Evening in play?

Hidden Gibbons rulings say and I guess it is applied to each case no matter how it got to be an enchantment:
4/10/2004 When it turns into a creature, it is no longer an enchantment.

It's not entirely clear how the one-shot effect that makes it lose all other types interacts with the continuous effect saying it has an extra type. I'd assumed that whichever of the two has the later timestamp would matter. Time to go hunt up a judge.

I can't easily test from where I am currently; how does it work with add-type-artifact effects? Say, Ashnod's Transmogrant for an effect-based one, and for an attached equipment or aura, Silverskin Armor? (Do we have that? Transmogrifying Licid is the alternative, and it seems like a poor choice to use for testing something other than itself...) The only add-artifact-to-everything card is Mycosynth Lattice, and I know we don't have that.