Hobgoblins' eye changes based on whether or not it can use Thermal Barrier. I've killed well over 10,000 Hobgoblins and yet not once noticed this small detail until I was just mucking about with the HUD off.
[spoiler]Left: On cooldown
Right: Active[/spoiler]
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I kind of like when they go into invincibility mode, from all my experience with armor lock in Halo Reach. Once they come out of it, they see me staring down the barrel of a shotgun at point blank range before they're obliterated. It's nostalgic, really.