This seems like one of those problems you solve in math class which has little application to the real world.
Exactly. For a real world example, If I have a flush, I'll often stop raising when the board pairs up. There's a theoretically mathematical remote chance the other player has the boat (~2-3% as mentioned by others), and you would think you should raise,, but in real life, I would say don't raise, especially with multiple players.
This is due to the effect that the players are in the hand for a reason. Similarly, if you have KK's in your hand, and and Ace shows up on board, your usually toast.
Not always. I had kings, made a big preflop raise and got called by A8. Ace came on the flop. A8 checked to me, I made a big c-bet, got called. Turn was a blank, fired another c-bet and got called. The river was another blank, again I was checked to, I put him all in and he folded. Showed me A8 and told me that he put me on a bigger ace. And he got on tilt when I showed him kings. I would have folded if he reraised rather than just called, since there was a straight draw out there.