For we are the Music Makers, and we are the dreamers of the dream

[Glade Riven]

While I am using the word "cheat" as the base principle of the 3rd rule of Dungeon Mastering (Win if you can lose if you must, but always cheat), it may not be the most accurate of term for what is going on.

A good dungeon master is a Magician. And I don't mean like a wizard in whatever game you are playing. At the table, it is about putting on the interactive show. So sometimes what you plan isn't working out the way it needs to. Pull some strings, make the puppets dance, and when everyone is focused on something else, make your move. Sometimes it is adjusting a number on the sheet. Sometimes, you need more minions to show up to investigate the ruckas because the battle is too short. Sometimes a few bad guys need to wet themselves and run away. Sometimes the ogre needs to be taking a dump behind a tree while the goblins are getting themselves slaughtered. Sometimes when the players are intentionally being stupid, you have to make certain things impossible. And sometimes you need to employ a +4 Clue Bat of Rusty Spikes.

And if you do things right, none of your players should be the wiser. Heck, that's why they make DM screens.

 

[Mark CMG]

A DM cannot "cheat" since that would assume there is a goal of winning, which there is not, because the DM is not playing in the same way that the other players are playing, nor do the rules apply to the DM in the same manner as it does to the other players.