When the last two cards on the board happen to be pair.
When the last two cards on the board make a pair.
This occurs when the fourth street card (turn) and the fifth street card (river) happen to be a pair, but do not match any other community cards.
Two cards of the same rank that fall consecutively, usually on fourth or fifth street in Hold 'Em or Omaha
When the river makes a pair for your hand.
When the turn and river card are a pair.