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Guys, the card is actually working for the colored mana part : if it's untapped, it actually reduces the cost of the first spell of the selected subtype you cast by one colored mana and auto-tap to prevent abuse. I further improved this part to prevent AI from abusing it, however it's still an approximation and won't leave you the chance of of chosing if you want to use the Cavern or not.
You should consider that :
1) There is now way of my knowledge to add "uncounterable" to any spell on the stack, unless it's embedded in the card's code, like the cards you mentioned. When a spell is on the stack, you couldn't alter it in any part unless it's done from inside the card code (like the cards you mentioned do).
2) There's no way to restrict the use of the mana in pool, unless it's the hardcoded stuff for Mishra's Workshop.

Thank you for satisfying my curiosity!

Describe the Bug:
I have a Captain Sisay in play, and the AI has another one under the effect of my Oubliette.

The AI destroys my Oubloiette, but the two Sisays don't destroy each other despite the legend rule.

Which card did behave improperly ?
Captain Sisay and/or Oubliette

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
Kamigawa, EDH

What exactly should be the correct behavior/interaction ?
The legend rule should apply, and the two Captain Sisay should die.

Are any other cards possibly affected by this bug ?

Maybe other legends under oubliette.

Describe the Bug:
I have cast a Confiscate on AI's Sword of Feast and Famine, but I can't use the equipment. In fact, the AI's Slith remains equipped, and I cannot unequip it.

Which card did behave improperly ?
Confiscate and Sword of Feast and Famine

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
Kamigawa EDH

What exactly should be the correct behavior/interaction ?
I should be able to use the Sword of Feast and Famine as I like if I control it.

Are any other cards possibly affected by this bug ?
Maybe other stolen equipments.

It is definetely an Oubliette bug which is hardcoded and causes other strange bugs,too.

Only if the creature which was equipped with Sword of Feast and Famine dies the confiscated Sword of Feast and Famine allows you to equip another.

AJ is right, and since Oubliette code is really complex, this will remain unfixed right now.

Hardcoded stuff is hardcoded. The "steal permanent" stuff is still largely a mistery, so I really don't know how to fix this.

Describe the Bug:
Godo,Bandit Warlord after it attacks for first time doesn't get untapped and no 2nd attack phase begins.

Which card did behave improperly ?
Godo,Bandit Warlord

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
ML3.0 15/7V2

What exactly should be the correct behavior/interaction ?
Godo,Bandit Warlord

Are any other cards possibly affected by this bug ?
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So my opponent attacked with some big 7/7 worm token creatures, and he had an Eldrazi Monument in play which should have made them indestructible. But I was going to die anyway so I thought I would try my Chastise on them just to see what happened. It didn't kill them but it removed them from the attack window and just left them tapped and not attacking, which is a bug I think.

heres a savegame of it

confirmed, though I don't think 2 combat phases in a row are doable.

Might be due to the indestructible code, which seems to "hard reset" a creature but cause also those bugs to happen.

The "untap" effect now works, but you don't get the second combat phase, strange, for other cards works... I fear it'll remain this way.

Actually the Indestructible code "auto-regenerate" the creature, so this interaction is possible. And I don't know if it's fixable