Aclazotz/Temple of the Dead
While playing on PC my opponent's Aclazotz what able to transform from its Temple of the Dead land form back into Aclazotz while I had 2 or more cards in hand. Because of this, I lost a match in a Sealed Event.

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Drexelfiend#76028 commented
I had 4 in hand and my opponent was able to activate. I reported the player for exploting.
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GoodyPhil#71064 commented
Also had this experience, but from the side of the activator. I got my opponent down to 0 cards in library and activated my temple of the dead for fun, not realizing they had 2 cards in their hand.
EDIT: oh didn't realize either player could have 1 card in hand to make this work. I had 1 card in hand, not a bug!
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[Deleted User] commented
And how many cards did your opponent have in hand? Only one player has to have 1 or less cards, doesn't have to be an opponent.