- Halo: Combat Evolved:
- In "343 Guilty Spark" you encounter a Marine driven insane by the (then-unencountered) Flood who will slowly shoot at the Chief when approached. His shots are too slow to do any real damage, and he can be left alone or killed with no consequence.
- In only one room in the final level do you encounter cloaked Flood.
- Halo 2:
- "Gravemind" had the "Honor Guard Councilor", which was actually a glitched Zealot Elite. This is the only Zealot fought on difficulties lower than Legendary. This is also the only level where Spec-Ops Grunts and Elites appear as enemies. Likewise, an Elite during the final battle can be seen using a Fuel Rod Gun.
- "Quarantine Zone" has a single Sentinel that shoots with Needler rounds instead of the Sentinel Beam. According to one of the programmers, it was a rejected concept they forgot to remove.
- "Cairo Station" has a pair of Elites dual-wielding plasma pistols at the beginning, an uncloaked Stealth Elite, and an Elite wielding both a Plasma Rifle and Needler at the end.
- "Cairo Station" also has an unusual case of a Unique Ally. In the first hangar, you encounter the "Bloody Marine", a Marine that wears less armor than usual and is covered in blood. Despite his unique model, he is of absolutely no relevance, has no unique behavior, and no other Marine appears in that state anywhere else.
- At the end of the underwater passage on "Regret", you encounter a group of stealth-cloaked Grunts, which are never seen again (as enemies).
- In "Outskirts", you encounter a convoy of Shadows. Despite receiving an entry in the game manual, it is undrivable, only found in this level, and overall acts more as a set piece than an enemy.
- In "High Charity", in the Prophet's inner sanctum, some cloaked Flood will spawn on higher difficulties. In addition, a single Flood Combat Form carries a Brute Shot, while a few others carry Brute Plasma Rifles.
- In "Delta Halo", some Jackals carrying Plasma Rifles are encountered at the second structure.
- The Marines in "Sacred Icon", who are indirectly encountered. You can still fight them from across the pit (and grenade-jump across). A trigger kills them if you go around the corner.
- Halo 3:
- In "Tsavo Highway", on higher difficulties, some sniper towers contain Brutes with Beam Rifles. If you THOUGHT Jackal Snipers were terrifying...
- At the beginning of "Halo", a Flood Brute Combat Form carrying a Gravity Hammer is dropped by a dispersal pod. A Flood Marine Combat Form armed with a Rocket Launcher appears during the standoff outside the Control Room. Going through the last tunnel of the Warthog Run, some Flood Elite Combat Forms appear on one of the raised platforms with Fuel Rod Guns. These are the only times Combat Forms spawn with these weapons.
- Halo 3: ODST has an NMPD officer who will turn on the Rookie because He Knows Too Much on "Data Hive"note . This is the only time in the entire series that you have to fight a human enemy. "Data Hive" is also the only time in the campaign where there are gold-colored Drones, who are apparently adolescent leaders. Only one gold Drone appears, however.
- Halo: Reach:
- In every level of this prequel, there is a special hidden Elite who will randomly appear out of nowhere in certain areas if you're fast enough in playing the level. These Elites, nicknamed "BOBs", give you a load of points when killed (to add to your credits if scoring is turned on) and a special medal if your Xbox Live account is linked to Bungie's site, but if you don't kill them fast enough they will disappear. BOBs come in two flavors: Gold Spec-Ops Rangers and completely white, random model Elites, similar to the aforementioned Honor Guard Councillor. The second variety's white coloration is due to a spawning error where the game "forgets" to add the correct color to the Elite character model.
- The Elite Field Marshal is the only one of its rank in "Pillar of Autumn". It is basically a high-ranking Zealot that rapidly fires a Fuel Rod Gun and throws grenades at the player. When its shields are down and when the player is close, it unsheaths its Energy Sword in an attempt to One-Hit Kill the player.
- There's one point early in the game where you fight a pair of King Kong-sized lizard-gorillas (referred to as Guta), who are basically part of Reach's local wildlife. They're tougher than Covenant, but not so tough that you could call them a boss battle or anything. They're never seen or heard of again afterwards, making the whole encounter something of a Big-Lipped Alligator Moment.
- Similar to the BOBs, the first level contains a lone Stealth Elite (different color than the cloaked Spec-Ops Elites encountered later) nicknamed "Bill", which can be spotted just before the moas first appear. If killed, it drops a data pad that can be read. This also may be a leftover from a dummied-out objective.
- In "ONI: Sword Base", one room contains a group of Grunts carrying Plasma Rifles.
- In "The Pillar Of Autumn", a Jackal with a Plasma Rifle is in the room after the shipyards.
- "Long Night of Solace" is the only level that features Space Banshees, Phantom Gunboats, and Seraphs.
- In Halo Wars, the eighth mission Anders' Signal has Brutes (as well as Choppers), the only time in the campaign where they appear as enemy units. They are fought at the beginning, and their introduction is completely overlapped by the subsequent introduction of the Flood.
Unique Enemy / Halo