Apparently it was in the manual, but I'm just learning it now.

Gamer sites on the early Internet were full of these "Easter eggs" that were really just non-obvious things with clear explanations in the manual.
One that I found particularly irrimusing (and seems to keep popping up forever) was that holding some combination of buttons on the Gameboy Advance when you turn it on "plays a secret, alternative start-up sound, then it just sits at the Gameboy logo until you press a button. That's all it does."
Except if you read the manual you'd know that holding that button combo overrides the normal start-up and forces the GBA into multi-play download mode, so you can play those games without having to take the cartridge out of the console. Pressing a button in that mode cancels it and resumes normal start-up, loading a game from cartridge if one is inserted.
I've seen some people insist that their manual didn't say anything about this, but I have trouble believing them given that it was written in the manual for the GBA which I bought on launch day.

Me when I learned that Minesweeper actually had logic and you’re not supposed to just click randomly.

Found this out completely by accident once after my sister and I played some Mario.
I had the 2nd controller still plugged in, and while shooting the ducks I stepped on the controller and the ducks moved differently.
From the on, every time someone wanted to play duck hunt I would grab a second controller and make it harder for them.
Bonus knowledge: the original game works by a light-sensitive sensor in the blaster tip, and when you pull the trigger, the screen goes black and a white square appears whee the ducks were, in a specific order. If the game controller detects the light square, it counts as a "hit" on whatever duck was in frame. You can cheat by pointing the blaster at a white light and pulling the trigger. It will just go through them one by one as you squeeze, thinking the light is the duck square.

I had Duck Hunt but didn't have the gun to play it with - nor the knowledge that I needed the gun. Every now and then I would try and fail to figure out how to play the game.
So to me, Duck Hunt is a game about a dog that laughs at you.
- This must have been a common thing, because you're the 2nd person in the comments to mention this!
  It's funny now to think that if you couldn't figure out a game pre-internet, you just didn't get to play it. I know that happened to me plenty.
  (edit: curse you, Batman on Sega Genesis!)
  
  Back in the day we would often rent games for a weekend and sometimes we would get stuck at some point. There was one particular game that me and my friends really liked (Maui Mallard) on the genesis, but there was one specific point we didn't know what to do. Every now and then we would rent the game again for a weekend in hopes of figuring it out. The game had basically three buttons IIRC: attack, jump and special. You could also press attack and special at the same time for a different attack. So one day I was playing it and reached the point that we all got stuck, and kept trying to figure out how to jump out of the area I was in (there was a clear exit, but too high up). My brother saw me struggling and mockingly said: "come on, do a super jump" and that made me think: can I do special + jump too? I tried it and then learned that this combination allowed climbing through short gaps (and this was the very first such gap in the game - anywhere else the combination did nothing). I was the neighborhood hero for a while thanks to that.
  
  We had tons of pirated Commodore 64 games. Some single-player games became two-player with my little brother; "OK, I'll drive the car while you try every key on the keyboard."

Gen Z/late millenials trying to interpret retro games they play on emulators with no manuals is the modern "people making extremely detailed marble statues because they don't realize Romans painted theirs".
That's how you end up with Blue Prince and Dark Souls and stuff.
- Tunic is really cool, it’s sorta based off the dudes experiences not being able to read and playing Zelda trying to use the manual to figure out wtf is going on, to my understanding.
- I refuse to believe that Romans painted theirs. I mean, the evidence is clear that they did but it would look so terrible!
  
  Yeah, and old games were just well designed with no handholding and absolutely didn't include full bullet pointed tutorials for the first hour in the manual as a matter of course.
- I'm interested in your take on what Blue Prince and Dark Souls are echoing, if I'm reading this right.
  
  Dark Souls is largely inspired by Miyazaki consuming western media without being able to fully understand it. He had to try to fill in the gaps himself. I assume that's what they meant.
  A good example of this is Tunic, where the manual is not understandable at first, but you can figure out as you play. These games create very interactive world building where you're supposed to pay attention and piece things together yourself instead of being handed the solution.
  
  I think they're saying if you fire up some old NES games without the manual, you'll only learn from trial and error, and it's going to be hard as hell. (Even with the manual, they were not as forgiving back then)
  Hence, people designing challenging games without instructions thinking THAT'S what the old timers must like!
- Wait, Blue Prince isn't just a point-and-click adventure/escape room kinda game?
  
  It is a mystery/puzzle/roguelike where you have to figure out what to do, how to do it, and even how the game works. You get a broad goal at the beginning, but you have to experiment, learn, and solve puzzles to progress. It basically requires that you take notes. It's brilliant
- Imagine not knowing about Vimm's manual project

The worst was games that required info from the manual to progress past a certain point, like star tropics. Rented the game and the rental place didn't include the manual? Shit out of luck. And no Internet back then to look it up, either. (Yes, I'm still bitter)
I remember some computer games would also do things like that to prevent copying the game from a friend, like requiring a certain word from a certain page before loading.
- I bought Sim City for PC at a used bookstore, and it didn't come with the reference page for a code it would ask you for after playing a certain amount of time.
  Without this code, the game would turn on all hazards (tornados, fires, flooding, Godzilla, etc) and make itself unplayable.
  
  Also it was black on red to make it harder to photocopy. I remember my mom being proud that she'd used the filters on the fancy copier she had at work to copy this sheet.
- There were rental places that didn't include the manual?
  
  One manual... where'd they get a replacement...it was like a library book, you had to return it.
  
  Most if them

If you press F in Skifree, you can outrun the snow monster.
- lies! my childhood refuses to believe this

This reminds me of the MGS one where the frequency for Meryl is on the back of the game case.
- Why not go with the direct example for MGS with switching to controller port 2?
  
  Because in the English version of MGS that's not "hidden" in the manual (or on the back of the box). You get the Colonel calling on the radio every ten seconds during that fight, virtually screaming at you "Hey you dumb kid, switch to the second controller port already!"
  
  Because it didn't remind me of that?

I didn't have the gun, but I had duck hunt, so I could only control the duck. Needless to say I didn't play much duck hunt

I feel like I was aware of this (much time has passed), but I think it's something we discovered by trying it out of curiosity.
- Back then, stuff like this spread by word of mouth somehow very effectively. I'd have a friend over and they'd just pick up the second controller and laugh when I missed the shot.
  There were a bunch of other things like the cheat code in Doom, the Contra code (although I think I saw that one in a magazine) putting the Warcraft 2 game disk into a CD player to get a secret audio track.
  
  The Blizzard songs were all also possible to play from the game install directory, too. Almost all of their early PC games had one of those. But yes, since they were a properly mastered audio track on the CD, they would play on regular music players too, which could be a really fun trick to show friends.

I've known about it since ii had the game as a kid in the early 90s because of the manual. It's def been in there all along lol

Til.

https://www.gamesdatabase.org/Media/SYSTEM/Nintendo_NES/manual/Formated/Duck_Hunt_-_1985_-_Nintendo.pdf