The Pico-8 version of Celeste inside of Celeste

The first example I ever have seen and my favorite was that Day of the Tentacle had a computer that would allow you to play the entire first game, Maniac Mansion.

In smash bros, turn the launch multiplier all the way up and turn on sudden death mode so everyone starts at 300%.

Go to one of a few levels (the underground area of Hyrule Temple works, the underground area of Skyworld is better, but it’s best if you make a custom stage)

Getting hit once will send you bouncing around the screen! You only ever die if you get unlucky. It’s hilarious, and we call it “Pinball Mode”. I’ve made a couple custom stages to improve on the experience.

My favorite was “Halloween” in Halo 2 multiplayer, kind of a knock off “hide-n-seek” but with a twist.

No shields, no radar, the only weapons were a sword and a pistol, and you only had one life. One player was “Michael Meyers”, that player had the sword and chased down everyone else. All the other players hid while Michael picked you off one by one. Once one player was left, that person became Jamie Lee Curtis and could try to take down Michael Meyers. (To be fair that person should be Dr Loomis or Laurie Strode but it was more fun to use the actor’s name than a character.)

The twist was the music. In Halo 2, you could hear enemy team chatter through the tv speakers and it got louder as you got closer to the enemy. Using that, Michael Meyers would have the Halloween theme song playing so you knew when he was close but you didn’t know where he was. It was truly thrilling, you hear the music, you know he’s close, but not where, the music would get louder and louder as he approached….and then he would walk right by your hiding spot. Or, you’re walking around as the music builds so you know he’s close, you think you’re safe because you have an idea where he is, and suddenly he’s right behind you with a sword in your back.

I play beam.ng and try not to crash.

Hide and seek when Among Us was big is one example I can think of.

Players get regular vision, the imposter got limited vision.

Also so good from back in the day it got turned into a forced standard on some servers: Down to 1V1 in CS? Knife fight about it

So fucking good. sometimes the bomb won. sometimes one guy was an asshole and shot the other guy

I am sure anyone who played Tears of the Kingdom did that at some point, but there are those koroks little guys you're supposed to guide to their destination, generally by designing machines to carry them there.

Of course instead you just strap them to the most absurd rocket powered contraptions and play Korok Space Program.

We used to play a "serial killer" game in Red Dead Redemption. The goal is to kidnap a woman without getting seen by anyone. Its actually a lot harder than you would think because female NPCs are RARELY unescorted in the game. It opens up an element of planning and stealth that you (or at least I) don't usually use in that game.

I played so much pong in commander keen 4.

The real game was too hard!

Playing a giant melee on super Smash Brothers Melee with super mushrooms on high spawn while using the biggest characters on the game and watch level is fun. I think it breaks the core game enough to qualify as unintended.

Child me played hide and seek in N64 GoldenEye with the help of construction paper and tape to cover the screen.

In GTA (the first one) we would come up with challenged like "steal a firetruck and ram 10 cars until they explode" and the other person had to do it with one life.

Demolition Derby in Halo.

No shooting, you get in the Hog and wreck the other fucker out of their rig, then run them over. Frag grenades sometimes legal, depending on who you were playing with.

Played a 4 player variant where the passenger got a rocket launcher-no gunner

Good Boy Galaxy has some in-game games including Poopsweeper (a Minesweeper clone), Fishing, Rhythm games, juggling and a Star Fox/Railshooter game

God, it's been years since I've done that.

I had Minecraft on the Xbox 360 when I was younger and would get up to all kinds of shenanigans with my friends, usually involving TNT. Like building the most impressive boat or castle we could, and then launching TNT at each other to see who could destroy all of the other players' cannons first. If we had 4 players, each team would have a bowman trying to slow down the person shooting TNT.

Once creative mode was added it was a game changer, but even before that point we would dupe hundreds of stacks of TNT with a glitch I can't remember (though I do recall it involved a furnace).