Cool Stuff

In System Shock and other shooters, the last bullet you have has multiplied damage.
Enemies in Bioshock will deliberately miss their first shot to give the players a chance to dodge.
Many platformers (I think Braid was one quoted) have a window where even if you fall off of a ledge, you can still jump.
Assassin's Creed and Doom have more health associated with the last tick of the health bar, to make you feel like you barely survived.
Shadow of Mordor grants additional health to dueling Uruks to increase the length of the fight for the sake of spectacle.
Silent Hill: Shattered Memories removed one physical sense of an AI every time you respawned in a nightmare run, slowed down enemies if you looked over the shoulder, and only tow enemies were allowed to chase you at once while the rest had to flank you.
Thumper's time signature corresponds to the numerical value of a level
Suikoden spawns less enemies in the world map if they're walking in a straight line while spawning more if you zigzag (the former is good for getting to a place quickly and the latter is for grinding)
Gears of War provided significant buffs to new players in multiplayer that tapered off with a few kills (to encourage them to replace multiplayer).
Half Life 2 has ledges and railings set as ragdoll magnets to enemies will fall over them more often.
Ratchet and Clank scaled enemy damage and hid enemies based on time played and total deaths of the player.
Jak and Daxter would trip players to mask the presence of loading
The Bureau/XCOM, enemy AI gets more aggressive if the players don't move every 15-20 seconds
In Thief: The Dark Project, your sword increases your visibility, meaning you need to choose better stealth or better preparation for being caught.
F.E.A.R bent bullets towards things that exploded
Enemies in some LEGO games have a hit or miss chance. If a projectile misses, it's offset and has no collision. This is done to make fights more hectic.
Alien:Isolation has the Xenomorph learn player habits (if the player hides in lockers a lot, it learns that)
The Xenomorph has 2 brains - one that will always know where you are, and one that controls the body and is given hints by the first brain.
Far Cry 4 reduces the damage and accuracy of NPCs based on how many are near a player.
Enemies in Left 4 Dead deliberatly target players the furthest away from the group or have had the least aggro.
Hi Octane displays different stats for different cars even though they all have the same internal stats.
Enemies in Arkham Asylum do not perform 180 degree turns so the player can be stealthy.
Elizabeth in Bioshock: Infinite throws resource to the player based on the player's current state.
The last phase of a boss fight in Furi has a lower difficulty and is more visually impressive
Guitar Hero rates you out of 5 stars, but won't give you lower than a 3.
Enter the Gungeon has the AI warm up. The longer a play session is, the harder the AI gets.
Good PC shooters mimic analogue controls as follows: holding movement key during a frame=1, pressing or releasing=0.5, pressing and releasing during same frame=0.25 1/2
Counters to your current class in Overwatch sound louder.
Spec Ops: The Line changed stuff in the environment suddenly to make the player question his perception.
Firewatch counts silence as a player choice in dialogue conversations