It was the lizard artiliary, and only a specific single unit.  I have yet to recreate the bug again.  I'm sorry that I wasn't keeping track of what equipment I had on. I'll pick up the gauntlets of rokkur and see if I can recreate the bug.

My last three attempts to recreate the bug have failed.  I think your suggestion that it is very rare is spot on.  All attempts made with gauntlets of rokkur have been negative to recreate.  As you suggest I also do not feel that it was equipment that caused the bug, but rather a series of events that may have occured while I was fighting in the centre of the map.  Of course, if it happens again I will notify you and attempt to present more data.

Seeing that mines are buggy, I'd think the catapult was standing near a mine and your hero was trying to shoot the mine. It happens alot with towers.

I hadn't considered mines, but I also do not know the standard amount of health for a mine.  I did observe that Regulus at a high level continued to fire at the same spot (and not in a shot ground kind of way) and when selecting alternative targets would auto select that spot again after the new target was destroyed.  If these spots were mines then, as suggested, it would explain alot, but then why did the artiliary lizard not set it off, and why could I not kill them?  These objects were the target of extended attack, longer than most NPC's should endure.

Again, if I can replicate or experience this bug another time I will try to observe as much as possible and report the specifics.