An incorporeal creature (a specter) is standing next to ‘fighter A’ , adjacent to a wall.
The Specter moves into the wall, moves in the wall away from ‘fighter A’ 30 feet to be next to ‘Mage B’ while still inside the wall.
The specter attacks ‘Mage B’ (with the 50% miss chance, since he can’t see ‘mage B’ ) from inside the wall.
Does ‘fighter A’ get an attack of Opportunity on the specter?
I ruled that it was since the square the specter moved into (and proved AoO was solid stone, which the fighters sword couldn't penetrate.
This generated a spirited discussion around the table. Thoughts?
Bad call.
You provoke the AoO when you LEAVE a square. Reading your story, it looks like the spectre was completely out of the wall in an empty square adjacent to the wall (e.g. it had no cover from the wall and presumably from nothing else since you didn't mention any other source of cover). So when it left that square into the wall, it provoked, and since the moment it provokes it is still in its starting square, not yet in the wall, the fighter gets his AoO.
For further consideration, imagine if the wall was not there at all. Imagine your spectre was just an orc. If the orc made the same movement, would it provoke? Of course. Now if that fighter had a longsword (or any other non-reach weapon), would he be able to take that AoO? Yes. But he can't reach the square the orc moved TO, so he must take that AoO in the square the orc moved FROM. Which is RAW. And that applies to the spectre leaving the threatened square in the same way.
An incorporeal creature inside an object has total cover...
And
You can't make an attack against a target that has total cover.
However, the specter moved from a space of non-cover while in the fighters reach - thereby leaving a threatened square. The fighter gets an AoO when the specter starts its move BEFORE it completely enters the wall (AoO's go off before the triggering action - the triggering action here was the specter leaving a square threatened by the fighter).