Helldritch
Hero
1) IF an encounter is deadly, players will definitely know in advance.But what if you realize, after the encounter begins, that you (the DM) inadvertently conveyed incorrect information to the party or otherwise made a major mistake on the actual encounter? That the group would have acted very differently if they had been conveyed the correct information.
Such that, and encounter you telegraphed as, and intended to be, easy (and that the group engaged on based on the information provided) is actually deadly (for example)?
If you saw the mistake only after the group engaged, would you change anything?
2) IF I ever do such a mistake, I will start playing the foe(s) with over confidence and describe the damage that the players are doing as insignificant. This is the clue that characters should flee ASAP. If they don't, well, it would not be my first TPK. And all players know that whenever I do this, it means flee or die.
3) If the fight is deadlier because of a streak of real bad luck on the players and incredible good luck on my side, I will simply make tactical errors with the foe(s). If this is not enough, fleeing is always an option. If they don't, well, it would not be my first TPK either...
But TPK are relatively rare. And so far, usually because the group decided to do the "Do or die" stuff even if it was not mandatory...