Invisible Hercules Class Titan in a Crimson mission.

[AlienMafia]

Just a random bug. No idea how to recreate.

[srmalloy]

I've had invisible mobs pop up from time to time -- the doors on the north side of the central building in Kallisti Wharf during the Halloween event were notorious for spawning invisible 'trick' mobs.

[Shin Magmus]

This sounds awesome.  Cloaked Robots should be a feature.