A few pages back someone named PanzerWolf posted about the Sidar hidden units being teleported when they engage in combat and also about a bug with a Great Commander being able to be joined by more than one Sidar hidden unit. I have taken a quite extensive look at this phenomenon and would like to share a few additional facts I have learned.
I have only looked at this with the Sidar, though I suppose it is possible that this might be happening with ALL hidden units. The problem is not limited to combat only. I have seen a hidden Sidar (almost always a ghost, sometimes a Shadow) get teleported away when exploring a lair (in this case, it was a goblin fort). Not only that, but I got the message "lair destroyed", yet the lair was still there to be explored again and again, sometimes again with the "lair destroyed" message. I have seen the hidden Sidar get teleported away one or two spaces when attacking, but in these cases since the "y" patch the unit then immediately moves back to where it should be. I have been able to reproduce the Great Commander exploit several times. (The hidden Sidar moves into the tile with the Great Commander, chooses the "join commander" option, gets teleported away with the Great Commander promotion, but leaves the Great Commander unit still in its original tile). This allows for a number of Great Commander promotions limited only by the number of hidden units you own. Further, each unit can later "Split" from its Great Commander (just before shading), creating a new Great Commander unit which can then be joined to any non-hidden unit.
It gets worse than this: The same bug that occurs with the Great Commander will occur with ANY equipment. Imagine every one of your ghosts with its own Nether Blade, then dropping those blades in the city it joins when shading, leaving the blade behind for any other unit to pick up.
With further testing I have shown that this does not occur if the unit is NOT hidden, but if the unit is hidden it will occur every time, no exceptions.
In the saved game below, you can see in the city Celo both a Great Commander and the Nether Blade (I shaded Rathus as soon as he reached level 6, which happened rather quickly as he had......wait for it.....a Great Commander promotion!) And next to Celo are two ghosts, both with the Great Commander promotion AND a Nether Blade. Until these bugs are fixed, it will be possible as the Sidar to maintain an entire army of ghosts; each with the Great Commander promotion, each carrying its own Nether Blade and whatever other equipment you may have acquired (Rod of Winds? Scorched Staff? Both?).