But what is an exploit?

[Scurvy Cur]

Things

Were any of this true, then the other spell which provides dodge AC, mage armor, would also stack infinitely with itself. However, it does not. One casting of mage armor (the non-shadow variant) will replace previous castings.

Shadow Conjuration mage armor is unique and exceptional, and it is fairly clear that this is not an intended design feature.

[Nulstarius]

Most of the known old spell bugs have been fixed over time by now. It has been years since I pointed shadow conjuration and the +20 ac potential out to Mithreas, dude snaps his magic fingers, next day it's fixed. But some bugs you don't go posting on the forums when you find them, you send them in nice and quiet.
So no need to worry about most the classics.

[HurinWillSmite]

Never underestimate the patience of an exploiter when he is exploiting.