It happens very often that you think you are ahead in the hand or your hand has good chances to improve to a winning hand, while in reality you are already behind with little or no chance of winning. Example: Yopu hold AKo, your opponent holds AA. The flop comes AK7 rainbow. You think you are ahead, but the truth is you need two Kings on Turn and River, because your opponent will otherwise have the better 3oak or full house. This is a typical case of drawing dead. Your hand is dominated by your opponents hand. So, when calculating the outs, you must never forget that a card that improves your hand might actually improve the opponents hand even more.
Typical “drawing live” situation: The Opponent has QJo, your hand is AKo. Flop comes J73 rainbow. Here all Aces and Kings are outs (6 outs).

When you hit your card on the turn, you can still loose on the river if your opponent gets lucky. Say you hold AQh and the flop shows 9h 7h 7d. You have a nutflushdraw. The turn brings 2h. Your flush has materialized! You bet, but your opponent calls. The river card is 9c. You bet again, but now your opponent raises you. It turns out that he had two pair on the flop (Nines and Sevens) and the river gave him a full house. Nevertheless it was right to bet on turn and river. But after the opponents raise on the river, you should not re-raise but just call if there is a pair on the board.