Discovering how to get to the first alternative path in Lylat Wars and thinking I was a genius.
My dad hiding my copy of Pokemon Red on Xmas day, then when he tried to reveal where he'd hidden it, it wasn't there and he thought it had been stolen. Turns out my brother had found it the night before and put it back with the rest of the presents under the tree, and it had fallen behind the box the tree was balanced on. I then went into my gaming cave (IE: the shed we converted into a game room with a bed and door to the outside toilet) and didn't emerge for over a week as I'd already taken the liberty of restocking the fridge.
Finally finding my way into the Black Hole level on Starwing and knowing I beat my dad there. He got me into Starwing. In fact, on the easy path variation of Corneria, when you fly to the right of the big red block to get the hidden bomb, my friends still refer to that as the 'snowy bomb', my dad's nickname because he found it first then taught me how to get it.
Discovering Final Fantasy through a swap with one of my brother's friends. I loaned him Pokemon Red (I'd already finished it) and he loaned me FF7 in return, the idea being we swapped back when both of us had finished the new games. I still have that same copy of FF7 today. It's less than five feet away from me right now. Still working after over a decade of abuse. Needed one major repair though. It got scuffed by a dodgy PS1 and I was saved on Cactuar Island. Cloud wouldn't appear on the world map, you could fly the Highwind but everything except Cactuar Island was black and crashed the game, and going into battle crashed the game. I thought I'd taken too long beating the game and Meteor had hit or something. Then I ran a disk cleaner over it I got at a car boot sale with a special scratch filling cream and it's worked perfectly ever since. What I wouldn't give for that miracle cleaning kit again.
My dad's girlfriend stealing my exceptionally rare and valuable Zelda game watch. Never did get that back. Nor did I get Wild Arms or Legend of the Dragoon back from that thieving skank-ass whore either.
I have more but I'm starting to ramble...