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Describe the Bug:
The engine won't let me choose a legal target enchantment. At least it looks legal to me.
Also, I should be able to go on and destroy everything else, even if there were no legal target enchantment. At least that's how I think it's meant to work. As of now, I can only cancel, give up the casting and lose the mana.
EDIT: it cannot even be cast if there's no "targetable" card of one the listed types. The first time it let me go on with the casting since it identified my own enchantments as valid targets, while rejected the enemy's as valid targets for some reason.

Which card did behave improperly ?
Decimate.

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
September 2013, 3.0, Duel

What exactly should be the correct behavior/interaction ?
Select the target and/or let me go on with next target type. I assume.

Are any other cards possibly affected by this bug ?
Any card with multiple target types selectable, of which some might not be represented by actual valid instances on the board?
Tricky.

The second bug you report - not being able to cast unless all targets are available - is correct behavior. (There don't need to be four separate ones - the same animated Blinkmoth Nexus could be used for your artifact, creature, and land targets - but you'd still need an enchantment to target as well.)

Ok, makes sense for balance reasons, though the card's wording isn't very clear in this regard. Thanks.
Still, the first point remains. There were 2 valid enemy enchantments that I wasn't allowed to select (don't remember which ones, I'm not at home, but they're in the savegame).

Decimate in fact is allowed "by code" to target instead of enchantments creatures and instead of lands creatures as well.

Not just allowed, required. Fixed e948397.