That doesn't really count as proof I'm afraidThe absolute highest AC you can get, without any temporary spells, is 37. Want proof? Read my article about it here.
While this is amusing (and a bit iffy - depending on rolling 12? Two shields? pfff), reading this thread made me realize something - armor has a strange return curve.
Going from AC 12 to AC 13 is almost insignificant. It's usually worth it for PCs to get *some* kind of AC so they don't have AC 10, but that's because the efforts needed to do so are pretty minor (cast a petty spell, spend a few dozens gp etc). However, increasing your AC by 1 or 2 at those AC and sacrificing something big for it is usually not worth it. If a foe hits you 50% of the time, and now they are hitting you 45% of the time... congrats, you just reduced incoming damage by 1/10th... not bad, but not great.
However, once your AC becomes pretty good, 1 AC makes a pretty big difference. If an enemy needs 18 or higher to hit you, one more AC means you've just cut incoming damage by a third!
*however*... if you have AC 30 and they must roll a 20 to hit you... getting AC 31 doesn't help you *at all*, you are still hit on a 20! So going overboard on defense is not the best way to go![]()
I'm curious. I know that as AC increases the expected damage from a monster at a particular attack bonus greatly decreases. However, some monsters have a lower attack bonus than what we are looking at. Once you have gotten their chance to hit down to only hitting on a crit then adding more AC doesn't help. So if a character had 30 AC then bumping it up to 31 doesn't help on a very large portion of enemies the character may face. That's one consideration I never see get taken into account for absurdly high AC's. Almost everything in the low 20 range doesn't have that issue as most mosnters have a +3 attack minimum.
It's also worth noting that if the foe has multiple options on how to deal with the PCs, targeting the unkillable looking tank with some kind of mental magic is a pretty obvious solution. AC 30, -1 to wisdom save...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.