On the Reserved List -- Wizards has committed to never reprint these cards.
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Knowledge Vault
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Oracle Data
, 
: Exile the top card of your library face down.
: Sacrifice this artifact. If you do, discard your hand, then put all cards exiled with this artifact into their owner's hand.When this artifact leaves the battlefield, put all cards exiled with it into their owner's graveyard.
Official Judge Precedents
You can activate the second ability any time you have priority, including in response to itself. However, you will only be able to sacrifice the Vault as the first such ability resolves.
If you activate Knowledge Vault's second ability, all that happens at that time is that you pay a cost of . As the ability resolves, you must sacrifice Knowledge Vault. This is mandatory. If you are able to do so, you discard your hand and put all cards exiled with Knowledge Vault into their owners' hands. If you aren't able to do so because Knowledge Vault left the battlefield or changed controllers before the ability resolves, nothing happens.
You don't get to look at the cards exiled with Knowledge Vault.
