Back in the days of Red and Blue, I owned Pokemon Red and my sister Blue. I had had the game for a while and had the 8 badges etc. My sister had about 4 badges but because she had chosen Charmander as her first Pokemon, she had a hell of a time beating Brock and Misty. I didn't know this at the time. One day, a thought hit me. If I start a new game on her Blue and get different starters, I can transfer them to my game so I have all three. Perfect! So I pinched her Gameboy one day when she was at school and for some reason I wasn't (can't remember why, it was about 6 years ago now). I started a new game on her system and went to transfer the starters to my game when the box popped up saying "We will have to save the game." And I thought.... oh.... this is going wipe her progress so I can get the 3 starters. My thumb hovered over the 'A' button for a few seconds before I pressed it and wiped out her progress.
I got what I wanted but at what cost? I knew my sister wouldn't know I had done it, she was 12 and ignorant and would believe what I told her. That her game was corrupted.
I put her Gameboy back and sat down with my 3 starters. After 10 minutes I stopped, put my Gameboy down. Said the words "fuck sake", went and got her Gameboy, and spent the rest of the day getting her game back to the point (more or less) that I had wiped it from.
I wish I had never wiped her game to start with as it made me feel like a tool for ruining her game just so I could be a little closer to "Catching them all".
Luckily, she never found out.