The entirety of Surface Tension in the original Half-Life. Gordon Freeman finally makes it out of Black Mesa... only to discover that the marines and aliens are still fighting, and the marines are constantly bombing the area. So he goes through, fighting, marines, aliens, helicopters, turret placements, and main battle tanks, all while the aliens and soldiers fight one another (and in a small CMOA for the Marines, they usually win). Particular highlights include the battle against the squad of soldiers, turret placements, and helicopter at the Dam, the area where the marines Osprey continually drops soldiers to kill a group of alien grunts, and Freeman calling in an airstrike on a Gargantua.
The very first Vortigaunt you encounter in the original game introduces himself by banging on a heavy steel door, making a huge bulge in it, and then hitting it again full force, shattering the door to pieces, before throwing lightning at you.
An Alien Grunt in Surface Tension punches a human soldier so hard that he launches him and breaks a concrete wall.
The subversion of Air Vent Escape. In Surface Tension, you're crawling through an air vent, like you've done so many times before, with the soldiers below chattering on their radios none the wiser. Then suddenly one of them says "I hear something". Cue the soldier and his buddy shooting the crap out of the vent, causing you to drop right out.
Another subversion occurs when you're crawling through a pipe (well, close enough to an air vent). Just when you think you're in the clear, a soldier opens the other hatch and drops a satchel in it. Better run like hell!
Half-Life 2: Episode Two had the first Hunter battle. Alyx hears the Hunters outside and gets all scared, having almost died from her last encounter with them, only to turn around and take a badass pill before commencing to slaughter them all.
Any time Dog gets into a fight, an event which will probably involve much car-flinging, flailing of trashcan-sized metal fists, and excessive application of ragdoll physics.
In Episode 2, he fights a Strider. He not only wins, but tears that building-sized heavily-armed monstrosity into pieces. By which we mean DOG tears the Striders BRAIN out.
Episode 2, when about sixteen million Antlions come pouring into the chamber where Alyx is being healed. Just when you're running low on ammo, the turrets are frazzled and the mines have run out, three Vortigaunts bust through the ceiling. One of them murmurs, "We will quiet them," and they turn to face the Antlions, revealing that for all their enigmatic proverbs and general butt-ugliness, Vortigaunts are fucking badasses. They proceed to unleash all manner of green-lightning-fueled fury on the Antlions, stunning them with bolts of electricity, frying them with concentrated beams, even judo-throwing them to the ground and exploding them with balls of energy.
Their actions in Episode One, where they temporarily free Freeman from the G-Man: Up until that moment, the G-Man was a Smug Snake with no apparent weaknesses, in constant control of the situation, and most of the cast didn't even know he existed. The expression on his face when he realizes he's lost control: priceless. "We'll see... about that."
The G-man does get get his own back in Episode Two, with his own CMOA. ("Doctor Freeemaaan. I realize this moment may not be the most convenient for a 'heart-to-heart', but I had to wait until your... 'friends' were otherwise occupied."), topped only by his Take That to Gordon regarding his actions in Half-Life ("There was a time they cared nothing for Ms... Vance; when their only experience of... 'humanity' was a crowbar coming at them down a steel corridor".)
Not only was of Freeman but of the Vortigaunts as well. G-Man's Episode Two speech is whole cutscene of awesome. The Vorts knocked the Smug Snake down a peg at the end of Episode One. What does G-Man do? The second he is able to pass the Vorts unnoticed, he gives his badass speech, reveals how he was the one to save Alyx from Black Mesa, and wants Freeman to pay up for his survival by bringing Alyx to White Forest, and gets her to relay a message of doom to Eli as a form of Take Thatfrom the man's own daughter. The closest G-Man comes to losing his cool is the occasional tone of anger expressed when enunciating.
Also; mowing down hunters while killing Striders WITH A PISTOL.
Alyx Vance: the scene in Breen's office. Alyx is currently held captive and immobile. She spits in Breen's face when he mentions her mother. Breen comments on Alyx's stubborn nature. Alyx: "You don't know the half of it."
From the perspective of the other characters, Gordon is a walking CMOA. Alyx would name the defeat of the gunship and then later the Strider in Episode One as Gordon's CMOA.
For further illustration on that, at one point in Episode 2, Gordon and a vortiguant are infiltrating an Antlion hive. The vortiguant feels the need to specifically tell Gordon not to kill the Antlion Guard, an insect monster the size of a truck that can throw cars around, laugh off grenades, and the glowing green stuff on it? That's a neurotoxin. About ten minutes after they get what they need from the hive, Gordon gets the go ahead and kills two of them, with little more than a shotgun and the trash that was laying around the valley.
And for even further illustration, the first definite objective Gordon is given in Half Life 2 aside from "escape" is to infiltrate a Combine prison and rescue a high priority prisoner, and no-one considers one man doing this to be unusual (granted he later receives a Red Shirt Army of Antlions, but they don't know that). After he succeeds, the resistance takes this as their cue to begin a worldwide revolution and as soon as Gordon appears they follow behind him unquestionably, because things in front of him have a tendency to die.
For an average player, the game can last in the area of 6 to 10 hours. This speed run completes the game in less than half an hour.
Storming Nova Prospekt. You are entering an an enemy base, while having an army of antlions at your hands. Plus, Apprehension and Evasion plays at the right time.
Barney gets a big one in Blue Shift: He actually gets out of the Black Mesa facility alive without the interference of the G-Man. That's something that Gordon and Adrian DIDN'T do. Not to mention that he does it without an HEV suit or any experimental weaponry, but with just a helmet and a Kevlar vest.
The very existence of Gravity Gun.
Pick any given moment from the original game out of a hat. Taking out the tentacles with a rocket, killing the Gargantua, destroying helicopters, getting past the trip mines, hell the entire Surface Tension chapter is one big ass CMOA!
If you want to give yourself a CMOA, it is entirely possible (through a combination of agility and persistence) to kill an Ant-Lion Guard with a crowbar.
It is perfectly possible to kill a hunter with a crowbar and not take damage.
The fan remake Black Mesa. Not only was it Saved from Development Hell, Valve and the community greenlit it to be on Steam for FREE. Clearly, Valve's been hiding their Half-Life related stuff for the opportunity to rise.