Folly said:
My reasoning steams from Artoomis' second point. Further, there is a point in your statement that I have an issue with. Your statement implies one of two situations: the dm is familar enough and fast enough with the rules that the players do not notice, or the players are so unobservant as to not notice changes in the combat. Both are really the same situation since it becomes relative between the capabilities of the dm and the capabilities of the players. Since there are 4 to 6 players (typically), the likelyhood of the DMs capabilities outstripping the players is unlikely.
Certainly, the DM has to know the rules well enough to pull it it off. Of course, DMing itself requires a certain minimum competence within the rules to begin with.
That aside, much of the success of DM "cheating", has less to do with rules knowledge, but with being able to decide which stat to change and when to change it by paying attention to what the players are paying attention to, having the self-control to not tell the players that you did it, and having enough composure to keep your poker face and not give it away.
For example... I've never run into a player who's bothered to keep track of all the hit point damage done to a monster by an entire party. Players generally only pay attention to the general amount of damage their character is dealing, and sometimes to the general amount of damage another character is dealing. They might know, "Wow, we've done more than 100 hit points damage, and it's still standing," but they generally won't no that they've done 114 total points of damage specifically. When the players are having a tough time, and it gets noticed, I may not even exactly subract hit points, but say to myself, "the monster goes down after three more hits, or the next critical hit, whichever comes first," and the players cheer and breath a sigh of relief at narrowly defeating a challenging foe... Too busy with healing and looting and victory celebrations to notice that the monster had 1/3 less hitpoints than it should have.
The point is, there are easy ways to cheat, and there tough ways to cheat...
Adding or subtracting hit points is easy... It rarely gets noticed, and is easy to explain away if it does.
Simply not using abilities is easy... Especially if it's a "hidden" ability like DR, or an ability the monster hasn't used in the battle yet. Other prominent and more often used abilities, like a giant squid not using Improved Grab might get noticed, however.
Changing AC is a tough one, if you aren't paying attention to what the players are doing... Players always pay attention to which attack rolls hit or miss. If a roll of 23 hits on one round, but misses on the next -- or vice versa -- it's almost certain to get noticed, and the only excuse you can fall back on is, "Sorry, I read it wrong."
All that said, I will agree with you on one thing... While nothing is illegal unless you get caught, the easiest way to not get caught is to not do it.
