I was at the very end of The Last Remnant with about 110 hours put in. I was like 2 achievements away from having them all and my power went out MID-SAVE.

Data got corrupted and I lost all that work. I had to replay the entire (110 hours) and get back to where I was. Which I didn't mind since I loved the game. I did so and got my 1000/1000G.

If they make the game for PS3 I would do it again.

When I gave my Snes to my little sisters (I was living in Playstation Country at the time and moving out of the house, too) and went back to play Chrono Trigger.  Those bitches erased all of my data for Chrono Trigger (every character except Magus leveled all the way up), Link to the Past (with my zero Death super speed run save), AND Super Metroid (with my speed run save)!!  They also kept erasing my Super Mario World and Donkey Kong Country saves but those were easy to replace.

And I bought a third party memory card for the PS1.  That think erased my data every week.  I was so pissed!

Try loosing a Final Fantasy VII save when I was about 3 quarters done of the game, twice! (this was before I ever beat the game)

I lost about 20 hours of gameplay on one of my oblivion saves.

I lost about 15 hours on one of my fallout 3 saves.

I lost about 40 hours of gameplay in GTAIV.

And never expect to have a game actually save and still be there on a SNES I learnt my lesson quick.

Oh, and Burnout for the PSP.  This happened fairly recently.  My fiance's nephew (Loves coming to my house so he can play videogames) couldn't wait until we got home to start gaming.  He grabbed my PSP and asked if he could play.  Of course, I said yes.  He decided to play Burnout Dominator.  I told him to play my slot and not to make a new one because he might erase my data.  I noticed he was sitting through the long intro instead of racing. Then he turned the PSP off and just sat there.

Got to work a few days later and tried to play the game. Dozens of hours were gone.  My d21lewis slot was gone--replaced with the name "Chase".

And not the same thing but:  I dated a girl back in 97.  She had a son so I brought my PS1 and my case of games over so he could play while I made out with his mom.  The kid went from game to game.  He was in gaming heaven.  Then he just stopped, went outside, and started playing basketball.  When I got home, I tried to play my PS1.  The damn part where you sit the disc (a little black circle) fell out and rolled away.  The girl SWORE her son didn't break my PS1.

Well, a few months ago, I was going to play pokemon Silver and check up on my pokemon that I've had ever since I was 10 (9 years ago) only to find that the data was corrupt. :(

My party was all LV100

The memory stick for my PSP got corrupted.  I lost everything including all my valkyrie profile: lenneth save games and haven't been able to bring myself to go through another playthrough.  Everything else I'd finished at the time.  I also gave away a computer that I'd forgotten had my FFVII save games on it, I've still never finished it despite having the PS1 discs now.

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. 

Had around 15 franchise seasons in Madden 2007 on PS2.   Would take a team with my roommate, play them for 5 years build them up, switch divisions/conference  and pick the worst teams in the NFL.  We played every game, playoff game etc.     Damn PS2 Memory card failed.  =/

Ever since my Tomb Raider save corrupted towards the end of the game on my PS1 I've always had 2 savegames on the go and alternate between them. To say I wasn't a happy bunny would be a major understatement lol

I hate to say it.. cause it's been 13 years and you may still have a relationship with her without this incident.. ;) but it f*cking happend all the time!! That frecking piece of plastic broke and broke all the time!!! And when it breaks it offset the balance and your cd's are scratched to hell!!! I went through 3 ps1's because of that untill I superglued that little m************ on it.. don't blame the kid.. blame Sony.. ( no sir that's not covered by warranty.. you shouldn't pull the CDs out that hard... blabla)