Yes. Sometimes 3 or 4 times a round. As soon as one of the extremely powergamed characters says "I hit AC 35" and I say "Miss", they immediately say "WHAT? I hit 35. There's no way that missed. How did it get that?"
And then I have to say "Well, it is a creature who started with a +12 natural armor. Then it got a template that adds a +2 deflection bonus. It is wearing +3 Fullplate designed for its size. Then it gets +1 from Dex."
Then they say "Wow...I can't believe you cheesed up the monster by adding a template and giving it custom +3 armor. Most of the monsters we fighter have around AC 25. It's a full 11 points higher than usual."
Because, if I'm off by one point, they'll notice. They'll call me on it. After all, the only way the game is fun for them and for me is if we play "fair". If the rules let you make something, it's fair. If it's just made up, then they might as well not play. After all, it's easy to beat them if you can arbitrarily make a monster AC 36. It's hard if they have to follow a set of rules to get to AC 36. At least, that's how they look at it. Or how they did look at it in 3.5e.
Now, in 4e, I simply say "It's a level 10 monster, it has appropriate defenses for it's level. There's no rules for what AC it can have. This one is higher than normal."
Wow.
1. 4e has rules for what AC a monster can have, there are guidelines in the DMG for ranges of AC based on monster role and level.
2 3e does not. It has rules on what things add to AC and in what ways, but no rules on how much AC to give a monster. There are no limits on how much natural armor a monster can have for example, only examples of what some exemplar monsters do have in the MM.
3 Majoru, your example is not that you did not play by the rules and therefore did not play fair, your players are complaining that you played by the 3e rules legally and they don't like it. They want you to stick to core MM stat blocks apparently. You played "fair" by your example of what you say your players required in 3e. 3e rules allow a ton of arbitrary flexibility while still playing by the rules.
NEw monster at AC 36. 10 base, +8 full plate, +2 shield, +16 natural. Done. Legal according to the rules of 3e. And they will only get non magical full plate and a shield as loot.
Pretty much this. I won't say that my players are this critical, but they all really understand the rules and have an encyclopedic knowledge of monster stats. As a result, if things do go outside the lines, they'll know about it.
Beyond that, what happens when your group sends a dispel magic over at your "made up on the fly" creature? How does that affect its stats. My group also had a spell from (I believe...it's been a while) the Spell Compendium that did damage to you for each spell that was currently affecting you. So I had to keep track of the number and kind of buff spells that I gave my creatures pretty much all the time.
In 4E, I can design much more to taste.
I know a lot of people must read a post like this and say "what the heck?" but if you say that you're likely not playing with a lot of "system mastery" type players. They do exist, and in my group's case are some of the best friends and roleplayers you'd ever want to meet.
What the heck?
Your players memorize monster stats but apparently ignore the parts about adding levels, advancing by HD, adding templates, and the ability of the DM to create their own monsters.
In 3e you can't say "oh its just an orc" with confidence. You don't know if it is the sample level 1 warrior from the MM, a level 20 barbarian, some type of 4HD monstrous humanoid mega orc variant race from a weird monster or race book or some black orc super unholy templated monstrosity from the Book of Templates Deluxe Edition 3.5.
My high system mastery face to face players know this very well.
I assumed most players with system mastery and an understanding of the 3e rules would realize it too.
Or that a DM who understood the rules could communicate that.