Hidden Stalker

The "Big and bad" mod contains a new npc named, npc_hstalker. The npc is much like a normal enemy but it fools you and mabye even gets away with it. It's scripted to go and disguise itself as a friendly. It'll shoot you if you damage it. It'll remove it's disguise into a form of a combine. It can trick you in some cases, did you kill it after shooting it? Who knows, the npc has smarter ai then a regular soldier and can preform battle-stradegies against you. Much like a bot with its ai in battle.