My only enemy is myself.
I deleted my Twilight Princess file just by saving over top of it with a newer one... when I was in the Palace of Twilight. And I hadn't beaten the game at that point. I was so close to finally finishing it and then I ruined it all just by force of habit causing me to save over the first file slot. I had to start over just to be able to claim I had completed TP. I have no idea why Twilight Princess thought it was a good idea to allow people to do that, since the other Zelda games never deemed it necessary before, and have not since deemed it necessary again. -.-
I also managed to delete my Super Smash Bros. Melee file, with HUNDREDS of hours of play time from me and my sister's childhoods, with all the characters and stages unlocked, just because I'm a complete and utter idiot. I was at a friend's house so that we could play together, but I forgot to put in the memory card before loading up the game. When the message "there is no memory card in slot A" appeared, I hastily put in the memory card. A new message popped up and I pressed A without reading it. Unfortunately for me, that message said something along the lines of "Would you like to overwrite the save data on this memory card and erase all of your childhood memories for no reason whatsoever because that totally makes more sense than just having the game automatically load it up for you if you forgot to put it in at first?"
Also one of my memory cards is now corrupted -- yes, the same one with Super Smash Bros. Melee. So the new file I restarted is gone now too. The game clearly doesn't want me to play it. Along with that went my first file for The Wind Waker, which was my first ever Zelda game, and my beautiful level 80+ Umbreon and Espeon in Pokémon Colosseum.
Oh, and this one time I accidentally erased my defeat of the second boss in The Adventure of Link because 3DS save states suck. Honestly, I don't even remember how it happened. I only remember that I was horrified that I would have to endure the torture of even REACHING the boss again. (Fortunately, I knew where to get the Life Spell the second time around, so it wasn't TOO awful...)
Save data is not my friend.
EDIT: Now that I think of it, I do remember letting this one kid play Ocarina of Time 3D on my 3DS and having him delete one of my save files... I was pretty mad at the time, but since I don't even remember what file it was, it probably wasn't my first one, nor was it that important. It must have been at least somewhat far in the game, though, or I wouldn't have cared. *shrug*