River Source!
 

River source mob in Warrens for the last part of Tunic of Ridossan quest isn't targetable, either by clicking or using /target or by cycling through closest NPCs. Tested from both sides of the water, in the water and all over. I can see the floating name in the water but can't target it.

Apparently mob can't be targetted but you can trade it by clicking it with the "harbringer of Bertoxxolous" in hand...except it ate the vial without giving me back the empty container as it should have.

This was fixed on our side in December and will be live in the next patch. Open a petition if you haven't gotten to it by now.