I was in the Water Gardens, and entered a "hidden alleyway" style explorable. Just to the left after the first rope bridge in.
Within was the Tchun explorable, with the large mob I wont spoiler.
I popped out, buffed the bejeezus our of myself, summoned all my courage for the epic battle, and reentered the transition... only to find it was now Lundros the Luminary with his ash refiner, instead!
I refined some ash and left, whereupon NwNs hard crashed during the transition.
On relog, I was back in the watergardens, and the explorable was still there. When I went back in, it was back to being Tchun's massacre.
I've seen some discoveries/explorables spawn ontop of each other, perhaps this was a similar issue.
Once you clear a discovery it 'goes away' on exit. Its quite possible that a DM spawned a lot of them and two were simply on top of each-other.
Has this been a frequent occurrance?
Was a one off for me.
I suppose it could have been a situation where there was one, another spawned on top of the first, but then ended when I exited, revealing the first one underneath again.... That makes sense.