Counterfeited would be, for example, if your opponent had the 23QJ in their hand.
So before the river you are ahead
A2456 and your opponent is behind with 23456.
After the river, and here comes the Ace on the board, you have been counterfeited. You already had an Ace and your hand does not change, You still have A2456, but your opponent now has won low with A2345. The Ace helped your opponent win low and the Ace on the river counterfeited you from winning, because you did not need an Ace since one was already in your hand.
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