In Outlaws, on the firts map of the game when you blow up the latrine with tree stick of dynamite and swim though the cave you can find the UFO with alians and a stolen cow.

https://www.youtube.com/watch?v=PKeEE5esh4Y&ab_channel=DasTasche

Wolfenstein®: The Old Blood - The Grammar Nazi. Best of the best.

The reference to the "Place That Sends You Mad" from the Asterix movie "The Twelve Tasks of Asterix" in the Witcher 3 Blood & Wine expansion, when you have to get Permit A38 during the Paperchase quest.

The Contra code, that everyone knows by heart.

Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start

In Metal Gear Solid 2: Sons of Liberty (and Substance), if you move fast enough, you get to see Solid Snake sneaking around in a cardboard box.

And if you are fast enough to shoot him and destroy the box, Snake instantly switches to a new one.

Go to the edge of any map in Motocross Madness or Motocross Madness 2. The game kicks you back into the map with the sound of a circus cannon, complete with the Wile E. Coyote falling sound and the sounds of cruching muscles and bones. Then, you respawn on the bike, none the worse for the wear.

It's not the deepest or most game-changing secret, but it did give me and my friends a lot of laughs when we we children.

As the rules ban obscene content, let us go with a literal easter egg instead:

Game: Futurama
Platform: PS2
Personally tested: no

On the Planet Express, you must first get Leela's keycard from her locker. Break the fire extinguisher window, but don't get the fire extinguisher. Destroy the TV while you have exactly 7 USD cash in hand.

You can get the cash as follows:
2 USD in the boiler room, 1 USD by opening a drawer in the lab, 4 USD upstairs.

JayFoxRox et al. found thie in 2019.

I mean, I still like the assassin easter egg in the witcher 2. It's hard to pick a favourite but that one's a contender at least.

holy hand grenades in( FALLOUT LAS VEGAS) reference to monty pythons holy grail!

For easter eggs my Favorite is still Might & Magic: Secrets of the Inner Sanctum and the Thundranium item. It was so funny because as a kid I watched Thundercats so seeing a strength giving consumable named that (which drained or killed Thundarans aka the Thundercats) was pretty comical.

I was Young and fiddling around with some buttons in the UI of "Raptor - Call of the Shadows" and was shocked I suddenly had some monkeys attacking me in the next mission (which I was not able to finish). My friends did not believe me until I could prove it to them.

I don't see anything in the rules saying you can only enter once?

So let us go with Breath of Fire II (SNES) also.

Game: Breath of Fire II
System SNES
Personally tested: no (but it is well documented)

In Bando, enter Manson's hallway. Stand on the arrow that points to the right for a couple of minutes to reveal a few interesting rooms, including one where you can get Seny, one of the six shamans, maybe up to two hours earlier than usual.
Feeling lucky? Well, Seny was programmed to increase your luck stat but due to a coding error, this doesn't happen (according to The Cutting Room Floor, at least).

What makes this secret interesting? Well, Breath of Fire II was released in 1994. This was found by op1282 and posted on GameFAQs in 2011. He took a bathroom break and was surprised to see the wall had moved again, but only found one of the five additional rooms he had opened, apparently.

So this secret took 17 years to find. But what distinguishes it from other secrets that took many years to find is that it has actual value for the player.

Not a secret or easter egg but in The Legend of Zelda : Phantom Hourglass, I came around a place where I found an object and from the look of it I said to myself "Well, I need to fold this..." then I remembered I played on a Nintendo DS, so I folded the console shut then opened it again and there it was, the little Zelda melody indicating you've solved a riddle. They actually must have detected the exit from sleep mode (as there was an alternate DS that didn't have folded screens).

That's the powerful thing to do when creating a videogame: making it leak into the real world...

White bunny in the Witcher 3. It was really suggestive. (Because I didn't have a single Holy Hand Grenade of Antioch in my inventory) For a quarter of an hour I tried stupidly to figure out how to get around it.

Not really a big secret, but during one of the many sub-stories in Yakuza 0 you meet Stephen Spining, who is in Tokyo to shoot a music video for the famous star Miracle Johnson.

If it wasn't obvious from the names, they're both funny lookalikes of Steven Spielberg and Michael Jackson.

Your role is to defend the king of pop from zombies as he moonwalks through the main street of Kamurocho, a very authentic replica of Tokyo's night life district Kabukicho.

Later on in the story you can meet Miracle again in an old school disco called Maharaja, where he challenges you to a dance battle. Needless to say he is very hard to beat, but one of the possible rewards he gives you after is very useful.