It was in Roarmulus Twins gate tonight, in the arena right above the boss levels. In the last wave of the first round, the Lichen Colony and a Scuttlebot were motionless near the middle.
And they ignored me completely, even as I kill them slowly with a gun. After I killed the lichen the scuttlebot was unstuck. It looks like they were running into each other and got stuck that way.
I've also seen other Scuttlebots running into walls too, but they turned around and attack when I hit them.
It's not uncommon for monsters to get stuck inside each other. It's quite easy to trigger with the Graviton/Electron series bombs.