My childhood had revolved around a lot of video game rumors. Mainly Pokemon Crystal which I had played an unhealthy amount. I think I even tried some Fire Red/Ruby/Red myths in it, like unlocking Mew. But the one that kept me awake the most was by far Celebi in a copy of the game without the use of any events or trades. I had actually gotten it working, as well. However, there is no way any living being could possibly witness the look on my face when I realized I could never catch it without breaking down into tears. You could only catch it with a Master Ball (only get 1) or a GS Ball (event only). I was a few years late to the Pokemon Crystal party, so I couldn't attend any events. I had wasted my master ball. You're not even able to spawn it without a GS Ball, so I guess seeing it should be enough.
In CastleVania: Symphony of the Night, my big brother convinced me there were 3 castles in the game. There are only 2, however. Considering the max percentage of completion in this game is 211.18 or something, it didn't sound unlikely to me that it could go further. My brother had to help me get the 2 rings required to go to the second castle, and wouldn't help me with the ridiculous thing he said I had to do to go to the third. The memories are fuzzy, but I think I had to beat one of the hardest bosses in the game (Nephi-something) Using only the Alucart armor and my fists. The Alucart armor is close to the most useless armor in the game, and I think it's common sense that fists are the weakest weapon in the game. I had grinded for potions to make the fight a lot easier, and eventually I think I actually did manage to do it, after which my brother told me that he was lying. I cried at that as well.
Considering this is one of my earliest video game memories, the details on it are probably really far off from what really happened, but the general idea is the same. In Legend of Zelda: Ocarina of Time, I convinced myself that you didn't need the rainbow bridge to cross over to Ganon's Castle. For these reasons, I absolutely avoided completing the Shadow and Spirt temple, as those are the last two temples no matter what. I went into them a lot for some of the items, mainly the hover boots from the spirit. I had tried really ridiculous things, like trying to plant bombs whilst floating with hoverboots to bounce off of them and reset the hover, landing directly in the middle of the lava pit using the exact same method, attempting to hookshot onto a stone wall for 2 hours believing that I was just doing it wrong. I also thought that if I somehow got to Ganon's Castle as a child, I would be able to float across.
One last one, this about Sonic Adventure. I was absolutely in love with both of the Sonic Adventure games forever. Shadow was my favorite character, and my brother told me that his friend unlocked Shadow in the original sonic adventure by completing every single Challenge A on every single level as every single character. This is near impossible, especially on some casino level, so naturally it took up my life for the next year. Or week, who knows. As far as I remember, I was never able to do this. I still can't even do it today, even when I try just for personal satisfaction.
If anyone wants more, I remember like 2 more. They're not as good but they're still about a really lonely 5 year old with too much free time.