cribbage puzzle

a staunch cribbage player has set me a challenge to solve the following puzzle. I have played myself leisurely for 30 years but cant seem to come up with an answer.

4 players put a card into the box.
each player has a score of 12 at this point.
when the top card is turned over, the score of each players hand is now 13...
what is the turned over card...??

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