A few years ago I was playing Animal Crossing on a non-Nintendo memory card for my Gamecube.
After spending oh about 300 hours, paying off my house and getting a whole bunch of stuff and 2 years worth of game play. I saw on the internet codes for NES games and money and what not. Being the excited pre-teen I was of course I wanted the codes.
So I run over to Nook's and tell him the codes and such. And I get a few games, money and I time travel to get more then 3 a day.
About 2 days later, I turn on my game and it says the save is corrupt. I was shocked.
I lost all my Animal Crossing data, LOTR data, Luigi's Mansion, SSBM and a bunch of other things. About 3000 hours I am guessing.
I have since learnt my lesson. My Animal Crossing file is now on a Nintendo memory card, and my other games on a new memory card. I do not time travel and only set my daylight savings time from the Gamecube menu.
I do not use codes anymore either.
Oh and then there was the time when my PC's HDD became corrupted and I lost 1000 hours of Age of Empires 2 campaign work.