The rare wasp, Volux (NPC ID 50895) phases out and becomes permanently inaccessible once you advance the zone quest chain past the point at which the bomb drops. For Alliance this is when you accept the quest Last Ditch Effort. It’s also an issue for Horde but I’m not sure what the phasing quest is.
This doesn’t seem to be intended behaviour, as it’s part of a batch of rares that were added later and were intended for general access.