In my youth, a time before the omnipresence of the internet, gamers would share secrets and advice. Some were true and some weren't, but there was never a way to know which were so you had to try them out. Here are some of the ones I heard of as a kid and tested out but to no avail:
Mortal Kombat 2- Play as Reptile
This was one of those school-playground legends that was actually true. Mortal Kombat was a popular game in my secondary school days, and challenging the mysterious 'Reptile' was a moment every gamer dreamt of. When you were fighting at 'The Pit' stage, certain objects would sometimes pass by the moon. This meant that you needed to complete the tough task of a double-flawless-fatality. If you were successful, a green, poison spewing lizard-man would appear and and challenge you- the fact that it actually just a Sub-Zero/ Scorpion a-like with green trim didn't matter. Doing this made you seem boss. I never managed it myself but I saw a couple of people who actually did in my local arcades/ sweet shop.
R-type- Hidden Level
I had R-type on the Master System and I heard about a secret level somewhere but never got to it myself. I'm not sure where I heard this rumour but I just looked it up on YouTube and now like vicariously through this amazing gamer.
Super Mario Bros- Minus World
This is an infamous glitch in Super Mario Bros. 1, which takes you to a never ending water level known as the 'Minus World.' I heard of this glitch through friends in the playground and my best friend and I tried in for hours but to no avail. But again, thanks to the wonder of YouTube I can once again live through others.