Saw it during Arcanaville's event in the beta server.
I accidentally spawned the non-combat version first, and then spawned the combat version for the players to defeat.
The next day on thursday, I logged into Beta and spawned a ton of stuff for players to attack. While circling the area outside of the Vanguard base spawning some Rikti AVs, who should sneak up behind me but the non-combat Nemesis Monstrosity, still wandering aimlessly around the zone. I killed him so players wouldn't try to engage him, and then spawned another combat version. A few hours later, it was gone so some players did in fact kill it.
