You’re the head judge of a Modern PPTQ, where Ace and Niño are playing their match. At the beginning of this turn, Ace untapped, put a counter on his AEther Vial, then picked up the top card of his library and looked at it. Thinking for a bit, Ace put that card face down next to his AEther Vial. After finishing thinking, Ace takes the new top card of his library and adds it to his hand. Niño immediately calls for a judge, and explains the situation. What do you do?
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Ace receives a warning for Hidden Card Error. Have Ace reveal his hand, but not the single card face-down near Aether Vial. Niño chooses one of those cards as an excess card and it is shuffled into the unknown portion of Ace’s library.