When a later card wipes out the value of your hand.
A card lands on the board and outranks your pair, so your hole cards stop playing in your hand. Example. On a board of 8-3-2 with 83 in your hand for two pair, if a K comes on both the turn and the river, the board's KK plays on top, and your hand is worth only a pair of 8s with a 3 kicker. In Hi-Lo it means a board card pairs one of your low cards so you no longer have the nut low, and with A-2 on a 3-5-7 flop, a 2 on the turn makes you lose to A-4. On the turn, 6 of 45 cards (13%) do this.