I have multiple games I attend. Most are quite fair, and I love them. However, in one of the games, we have a couple of players who cheat in various ways. One never tracks his spells, so after many battles he insists he still has a full arsenal of higher level spells to lob at the enemy. Another always holds his dice with his fingers after it has rolled, on the pretense of "seeing the number clearly." While doing this, he surreptitiously twists it to one of the adjacent, higher numbers. In our last game, he had to roll 6d6 for damage, but only had 3 dice. He rolled the first 3d6 fairly, then while the DM (me) recorded the damage, he "rolled" the next 3d6 by only rolling the one die with the lowest value, leaving the other two six-siders showing their nice high rolls. He genuinely tried to pass it off, thinking I was seriously that stupid.
I've caught him at it so many times and asked him to stop, and he never does.
It was at that moment that I decided that the battle was not playing out fairly, so I simply decided what the outcome would be and continued the fight until I had the expected outcome, which was a near-TPK. This was despite the fact that the party rolled almost all "natural" 20s for their hit rolls, and almost always got nearly max damage somehow for their unlimited use of spells. They're still talking about how amazingly hard those enemies were. It was almost... unreal how much damage they could absorb.
Don't cheat. The guy who is creating the enemies you fight can always compensate.
