It is true that on occasion a player may, accidentally, have an extra card in his hand. For instance, he may have been dealt 11 or 12 cards in error. He may have picked a card from the stock and forgot to discard, or sometimes two cards may be stuck together. In such cases, the rules of gin rummy specifically state that if either player’s hand is discovered to have an incorrect number of cards before that player has made his first draw, there must be a new deal.
After the first draw however, if it is discovered that both players have incorrect hands, there must be a new deal. But if one player’s hand is incorrect and the other player’s hand is correct, the player holding the proper number of cards has the option of either having a new deal or continuing play. If play continues, the player with the incorrect hand must correct his hand by discarding without drawing or by drawing without discarding, and may not knock until his next regular turn to play. That is, if one player has a card too many, he does not select a card from the deck when it is his turn to pick, but simply discards a card from his hand. If on the other hand the player is short a card, he may, in his regular turn, pick the last card discarded by his opponent or select a new card from the stock as in ordinary play, but he does not discard.
The choice of whether or not to continue play usually depends on the stage in which the game is in at the time the error is discovered. When it is discovered fairly late in the course of play that a player has an illegal number of cards, it generally means that his hand is nearly read for a decision such as a knock or a gin. Usually an irregular hand is not noticed until a player has at least six melded cards. Therefore, unless you are reasonably sure of winning the hand no matter what happens, you should have the hand replayed. Of course, if after your opponent has actually won the hand through a knock or gin it is found that he has an illegal number of cards, you should have the hand discarded or replayed.
The player who is deliberately cheating by dealing himself an extra card or by palming one can be a problem. This extra card will greatly enhance his opportunities for melds and the chances are a little better than average that he will win his hand before you discover that he has the wrong number of cards. In which case, he gets full credit for the value of his win.
To protect against a player who deliberately plays with an extra card in his hand, when knocking or ginning a hand make certain to follow the rule that requires that the entire hand must be turned face up as well as the card that is discarded. In fact, the hand and discard should be kept away from either the balance of the unused deck or the previously discarded cards. The loser of the hand must also lay his cards face up on the table. The most obvious way to protect yourself against this is to simply count the number of cards you are dealt as well as your opponent’s and to make sure to check them again during play to make sure you and your opponent both have a legal hand.