I've started playing through TLoZ: A Link to the Past again, and it reminded me of a case a few years ago where I made a discovery.

Of course, in 1992 and several years after, there was no Internet, and thus no game walkthroughs/guides. Finished LttP a few weeks after getting it in April of '92. Fast forward to December 2002. I got the GBA LttP port, and used a gamefaqs.com walkthrough. I was absolutely STUNNED to learn at the end of it that there was a level 4 sword that could be gotten at the end of the game by throwing your level 3 sword in the fountain and the fat fairy upgrading it!

So, for over 10 years I had no idea there was the level 4 gold sword, which makes the last fight so much easier! Embarrassing, but true! It must not have been a widely known thing, as I never heard of it in a game magazine or anything. Of course, I'd have found out if I threw different things in and experimented, but I didn't.

@FluttershyGuy pfffft! That's not embarrassing. You should have seen how long it took me to get the silver arrows. I fought Ganon so many times without em.

Both times I've beaten ALttP, I forgot to get the Silver Arrows.

Mario Bros. 3 World 8 there's a level where you can swim in lava. Normally you would die in lava just by touching it but if you look at the map, it's a lake shaped like a Koopa Skull. It's actually a blood pond.

This ties in with Japanese mythology about a blood pond in hell. I guess Bowser's Title in Japan, Daimao Koopa (Demon King Bowser) makes sense here.

Until earlier this year, I didn't notice that Giant Land from SMB3 is shaped like a Koopa Troopa.

I just realized yesterday that Bill in the Pokemon games is named after Bill Gates.

Only on the US version, the PAL version turns the number blue... and the Japanese version gives no indication.

That Wario Woods is the only NES game to receive a rating from the ESRB.

That the game Donkey Kong was originally going to be about Popeye.

I didn't know about going behind the scenery in SMB3 for years. I had no internets!

Me either! Well I had the internet, but I was too young to bother to look things up about my GBA.

@Retro & @RevolverLink
I just learned something too, as I didn't know Ganon could be beaten WITHOUT the Silver Arrows! So, all three of us did it the hard way originally. Me without level 4 sword, you two without silver arrows!

@Miss_Birdo
A blood pond? Boy, that's pretty hardcore for a Mario game! I'd never thought of that!

@CowLaunch
That's me too. Found out about dropping behind the white platforms in Nintendo Power or something. But, there's many years of games I realize I may not know everything about, because they were pre-Internetz.

Something I found out shortly after joining the Internet crowd in 1999 was that there's a sound test in Castlevania III! I liked that, because CV III's is music to my earholes.

Oh no, I mis-worded that. You can't actually beat Ganon without the Silver Arrows. I just meant that both times I beat ALttP, I forgot to get the Silver Arrows at first. After about 5-10 deaths, I had to actually to look up what the heck I was doing wrong.

Ah, I gotcha. I was thinking, fat fairy didn't know what she was talking about that those were the arrows needed to give Ganon his last breaths!

I didn't know about that until I played it on the 3DS (at first I thought it was new, until I looked it up on the intarwebs).

I also didn't know there was a blue rupee around the corner in the shop at the beginning of Ocarina of Time until I watched my GF play the game a few months ago.

Super Mario Bros. shares the same initials (SMB) with a lot more games than I thought. Even including the "Super" in the name. Super Monkey Ball and Super Meat Boy are two examples.

During my "SNES Replay" tour, I just made another discovery, this time about Final Fantasy: Mystic Quest (courtesy of GameFAQs). Benjamin can cast Cure on the final boss and take over 10,000 hit points off (rarely, 3,000 is the most you'll ever take off with an attack, with a weakness & critical hit). Yet your partner (who I'll avoid naming, as it's endgame) CURES the final boss by casting cure. PeCURElier, if you ask me!

Someone asking about the Blood Pond, it's pretty common in Japanese mythology and culture. There's also an actual pond in Japan. The Chinoike Jigoku (Bloody Pond Hell) in Beppu City. All the fire levels in Mario are named after Hell in the Japanese versions.

In Banjo Kazooie, I had always waited until getting the Golden Feathers in Clanker's Cavern before going attacking Nipper to get in his shell. I always thought that was how you were meant to do it, even up to when I played the XBLA version. I only found out you could just avoid his attacks after watching a Let's Play.

It always annoyed me because I would have to go back and get all the notes again when I came back.

Missingno's from Red/Blue/Yellow can be traded to Gold/Silver/Crystal. Thing is, they turn out to be actual Pokemon not made available until 2nd gen. The Pokemon in the orginal game are organized in the game's code by an index number, with the first being Rhydon and the last being Victreebel. Since the creator of Pokemon stated that Rhydon was the first Pokemon the team made, it'd make sense that this was the order the Pokemon were made in. Thing is, there's 190 of them, with 39 Missingno's sporadically dispersed through the game's code. When these Missingno are traded to Gold/Silver through the time machine, they turn out to be actual Pokemon. This could very well mean that many of the Pokemon in 2nd gen could have been made early in the development of Red and Blue, and later tossed to fit more towards the mechanics in the next gen of Pokemon, or to just make room for a sequel.
Here's the list of index numbers: http://bulbapedia.bulbagarden.net/wiki/List_of_Pok%C3%A9mon_b...
A member of the Smogon community (a big group of Pokemon battlers, Serebii is made up of them) was able to get an interview with Shigeki Morimoto, the creator of Mew, and the guy that put Mew in as a little thing for people to find, at the Pokemon World Championships in 2009. Here's how it goes out:

So at the worlds this year I decided to ask Shigeki Morimoto, the guy who made Mew, about it. Since I asked him at the autograph table, where he would have his translator, I didn't have long so I can pretty much outline the exchange as this:
Me: "So in Red and Green, were there originally exactly 190 Pokémon?"
Translator relays to Morimoto; he sort of jumps in his chair enthusiastically (lol) and seems very happy, grinning at me
Translator: "Yes; we decided to save the rest of the designs for later"
Me: "So for example Ho-Oh was made for Red and Green first? Since it was in the first episode of the anime?"
he hears Ho-Oh and looks confused, then then when the translator says the rest to him he remains the same
Translator: "No, not Ho-Oh. He wasn't made earlier."
And then I say domo arigato and get rushed out by the queue building behind me, heheh. However what he said about Ho-Oh just doesn't make sense so I'm guessing that was an error in the translation. Since Ho-Oh is #49 in the list and appears in anime episode 1 it obviously had been made by that time so he probably tried to say Ho-Oh wasn't made for Red and Green, and was just in there with no real intention of keeping it in.
So yeah, those are my findings! I hope you all find them interesting.

Here's where I found it: http://www.smogon.com/forums/showthread.php?t=78267
The beta art from the link also shows that most of the Pokemon in 2nd gen were pretty much made at this point. This also could help explain why the design of Pokemon from 1st and 2nd gen vary greatly from later generations. I'm saying that as a bad or good thing, but to point out that Pokemon before 3rd gen lacked complexity in their design. Nearly all of them made up only 1 or 2 colors overall, with black and white also in there. This was probably due to the graphical limitations of the Gameboy/GBC. When the GBA came out, the team could design pretty much whatever they wanted. That, and Ken Sugimori was no longer directly making Pokemon, but instead overseeing the creation of them.
I love the mystery behind so much of the 1st gen pokemon games. You could write books about this stuff.