We were finishing up a binosaur lair and got to the boss room. Nothing spawned when it gave us this objective. It was completely empty and unable to be completed.
NA - New York - Prime, I believe Reinforced Binosaur Lair 20-30 or so
As I said, we had just entered the boss room and it was completely devoid of enemies. We merely all gathered at the door and entered to an empty room which made the dungeon unbeatable.