Cool little hidden details or easter eggs in games that you discovered?

[Turbo Kid]

In stage 5 of The Messenger you'll notice that at certain points the mushrooms in the background are actually singing the level's music. Creepy considering that these are all people who were transformed into mushrooms and cursed to remain in these marshes by the level's boss. Also, if you listen to all of the Shopkeeper's stories you'll start to notice interesting things in the backgrounds of some locations that seem to indicate that her stories are actually not just made up BS.

There's a creepy Psychonauts reference in Alice Madness Returns

Years after I had stopped playing The Adventures of Batman and Robin on the SNES I saw a youtube video where someone played through the Catwoman stage and actually found her cat in a hidden room I never knew about.

 

[Waffles's iconWaffles]

There's a "future" Final Fantasy cover in Deus Ex: Human Revolution and, disturbingly, a picture of the Hindenburg going up in flames on Grim Fandango.

 

[Jayonnaise6105 Jayonnaise6105]

Discovered myself:

In the bathroom of D4: Dark Dreams Don't Die, there's a bag of "red seeds", which are a major plot point in Deadly Premonition.

 

[Mahan_Native]

soap in Ultrakill: there is a hidden soap in one of the ultrakill levels which can one shot every enemy.in fact this soap is a reference to a game named DUSK. that game also had soap which deals 1 billion damage.

Mario old clothes in SH2 Remake: In SH2 Remake, there is a room that Maria grabs a outfit from a closet which it's her OG SH2 outfit.

 

[RanmyakuIchi's icon RanmyakuIchi RanmyakuIchi]

There was the secret hidden room in Arkham Asylum that teases and calls forward to Arkham City. If I'm remembering right, it took a year or two for anyone to even find it.

 

[Bakuma1997]

In Akumajou Dracula: Boku Dracula Kun if you try the Konami code the game will mock you by believing it will work

 

[Jayonnaise6105 Jayonnaise6105]

Nier Automata:

Spoiler

Finding Kaine's shack from the first Nier deep underground, surrounded by a bunch of Lunar Tears

 

[jon2]

Dynasty Warriors 5 and 5XL:

Free Mode - Battle of Fan Castle (Shu)

In this stage Guan Yu is leading the Shu army, problem is

Spoiler

aside from the Wei army, the Wu army arrives as enemy reinforcements. This will cause your HQ generals, Fu Shiren and Mi Fang to defect to the enemy, enraging Guan Yu who will hunt down the two traitors. What Guan Yu doesn't know is that the Wu army, led by Lv Meng, has an ambush waiting for him; this is what kills Guan Yu according to the stage's story. His sworn brother, Zhang Fei comes late as reinforcements and will take over as main general after Guan Yu dies

BUT YOU CAN PREVENT THAT!

All you need to do is kill Fu Shiren and Mi Fang before

Spoiler

Guan Yu gets ambushed and dies

The easiest way to do so is to wait at the Shu HQ together with Fu Shiren and Mi Fang before the events happen

Spoiler

the sign is when the Wu army/Lv Meng arrives; at that point you must be at the Shu HQ or near it already

Once you properly kill Fu Shiren and Mi Fang, you will know you did it properly when Guan Yu says,

Spoiler

"Those traitors have paid for their crimes!"

Then kill all remaining enemy generals to finish the stage with Guan Yu still on your roster

 

[Segadude113]

Max from Sam & Max has a portrait in the past in Day of The Tentacle

He's also got a little cameo in Star Wars: Dark Forces via map screen

 

[Ferik]

A lot of people know that in Fallout 1, 2, and New Vegas if you play with a very low Intelligence character you get entirely unique dialogue where you can barely string three words together and everyone treats you like an imbecile. However, in 2 and New Vegas each there's one character who's also incredibly low intelligence, Torr in Klamath in 2 and the Vagrant in Freeside in NV, and if you speak with them as a low Int. character they suddenly become incredibly eloquent and well spoken, and treat you as a fellow intellectual.