Sometimes a card will come on the board that will hurt your hand because it makes previously valuable cards in your hand communal property. For example, suppose you hold A9 in a game of Texas Hold'em. The board is 5678. You hold a straight to the 9! However, if the final card on the river comes a 9, your hand is counterfeited. Now, everyone has a straight to the nine. Being counterfeited occurs a lot in Omaha Hi-Lo. Many times people's lows are counterfeited by the board. Suppose you hold AAK2. The board is 367Q. You hold the nut low (A2367). However, if the river comes a 2, you no longer hold the nut low. Your low is A2367 still. However, someone with A4 in their hand will have you beat with a low of A2346.
(Submitted by TwoGun)