Well for me it would have to be back when Final Fantasy VII was the popular kid on the block. I was only young but i loved that game. I must have spent hours each day slowly fighting my way through the strange lands. So like any young child I wished to share the experience with those around me, my mother being the closest candidate not already exposed. Now she is a very straight laced woman who doesn't really understand video games. (this same woman cant watch animation because to her its nothing more than moving pictures.)
After a long time of coercing her into it i finally convinced her to start a new game and give it a play through... she didn't sleep for 2 days and played constantly. when ever i walked through the lounge she would excitedly tell me about what she had been doing in the game. and then it happened...
finally able to put the game down she went back to reality, and a few days past. i went back to playing myself. and during the process of saving before a boss fight i accidently saved over HER save file. instantly i had realised what i had done but once it has been written over there is no going back. the next day she asked to play and i had to come clean. needless to say she reacted quite badly and after a rather loud and long rant on the pointlessness of video games she vowed never to play another single video game for the rest of her life. a vow that she has kept to this day.
so my case is that a single save lost had ruined forever this persons experience and completely turned them from gaming as a medium.
I've actually lost so many savegames in my entire life I would find it hard to point out or even remember which one was the worst. I'd say it would be Chrono Trigger, since recently I lost a very advanced save file for it (accidentally erased by me), but it was like the seven thousandth time I was playing the game, so no big deal.
A sort of related situation, though. A friend of mine was playing Resident Evil 4 and got to the final battle with Saddler. The final battle in the entire game. He couldn't beat Saddler (yeah, he's kind of not very good) so he decided to erase the save file and start again. That's right. He was in the final battle and he started the whole game again. I would have cried like a baby if something like that happened to me by accident and he did it on purpose.
This doesn't seem half as bad compared to everyone else's stories now, but still...
So, one random evening, my friend Pat shows up at our dorm with his xbox, and a copy of "dungeons and dragons", a hack 'n' slash adventure romp, which supported up to 4 people playing at the same time. Pat, his girlfrend Sonia, Joe, and myself decide that we're going to play through it. All of it. In one go.
Anyway, we start, and we push through it, and after a while, Pat and Sonia drop out, leaving it all down to me and Joe. We are not a very good team, and spend more time swearing at each other (creatively!) than actually killing mooks, but we're slowly getting through the game. We're a good way in, and we've gotten every piece of the Fancy Super Weapon.
It's around this point the sun starts coming up. We're near the area boss, and we go to grab the 15th piece of the Fancy Super Weapon. The little animation plays to show our weapon transforming... and it freezes. Completely. We reload, try it again. Autosave was lost when it crashed mid-autosave.
One day, Joe and I will go back, and find out what the Fancy Super Weapon looks like, and then stab the big bad with it/fill the screen with magic missile so I can't see what I'm doing goddamnit Joe. One day.
My computer just recently decided that it wanted to destroy my harddrive. So my 300+ hours of Skyrim. My 300+ hours of Fallout 3 and New Vegas. My Dragon Age Origins & 2 characters. And alot of other smaller games aswell.. All gone.. I could only sit there in silence and go ''Damn..''
Hmm, in Pokemon Yellow years ago I trapped myself in a cave somehow. I can't actually remember which one, maybe the one with Articuno in it. No Dig, no escape rope, and I had pushed the blocks around in a way that made escape impossible. 100 hours maybe? Gone. Devestating for a, shit how old was I? 9? 10? Somewhere in there.
Or there was Oblivion. Not a fully lost file. But I went from a 90 hour save to a 40 hour save because I reloaded to play a fun bit then got fucked by my own force-of-habit stupidity overwriting the most recent file. Incidentally, I did end up going back to that one and turning it into a 120 hour file. We can rebuild it, I thought tp myself. So we did.
I loved that game and counted down the minutes for months when I was young. It was the first T rated game I could convince my parents to buy. And once I had it I played it nonstop for months.
I don't know what it was about that game, maybe it was my lack of understanding of computers back then as opposed to now, but I had unlocked EVERY character, EVERY map, EVERY challenge, EVERY race-to-the-finish... everything.
Then I lost it.
The first time it happened it devastated me. Hey, I was 7 years old, what could I do but just unplug and replug for hours on end.
A couple years later... I got back on, rebooted the system, restarted from scratch, unlocked EVERY character, EVERY map, yada yada yada.... everything.
Then I lost it again.
Eventually, after years of gathering dust, and several house moves later, after I had gained a much better understanding of electronics, consoles, and just gaming technology and the industry as a whole, I was rummaging through old things in old closets for some components to help a friend build a computer, and in an old half-broken cardboard box, I found the source of one of my most painful childhood memories...
So I kicked its ass.
Now I have everything, and I mean everything, again. I'll hook up the old GameCube and beat down some Master Hand ass with Link or maybe Samus and hell maybe I'll faceroll with Marth when I'm angry now and again but mostly, I just use it as a trophy for overcoming my childhood enemy .
I know it's not the most dramatic or serious story but I'm entertained by it.
Most definitely Pokemon Gold when my cart battery died a few years back. I've since replaced the battery, but the loss of my 300+ hour save was infuriating...
Another that comes to mind is Onimusha 2. I was just about at the final stage, when I decided it would be fun to replay the part at the beginning in the village area. I went to save my new file as the second slot, but I wasn't thinking and I accidentally saved over my main file. I realized my mistake as soon as I made it, and I expected a confirmation "Are you sure? YES NO" message to come up... I used to take those messages for granted ):
Edit: I recently lost my Terraria world when my computer crashed. My friend and I spent a solid 36 hours straight building huge structures manually. I was so proud of it too...
I've accidentally saved over my save file at least once on each of the first three Ratchet and Clank games due to me wanting to start playing as soon as possible. I also fucked up a save of a map I was working on in Halo Reach. I had gotten pretty far on the save I fucked up and the only file that had any of the map left was too far back and had way too many things for me to attempt to fix to even try continuing on it.
My little brother also decided that it would be a good idea to get rid of all of the "junk" files on one of "his" PS2 memory cards. As you can imagine it was actually my card that he had written his name on (without checking first) and then started deleting files on it without even thinking. What should have given it away that the memory card was mine was that we had stored all of the Jak&Daxter games only on my card (which included two corrupted files of Jak X [sub]stupid fucking autosave[/sub]) and all of the files for games like the Star Wars Battlefronts that he should have known weren't his. Thankfully I came into the room before he deleted any more of my files. It should also be noted that this same brother has sold: most of the Ratchet and Clank games that we've owned, both Star Wars Battlefronts, Bioshock, Oblivion, and a few other really good games that I can't think of at the moment.
Recently, I was playing the Witcher 2 on hardcore. My mouse had been acting funny all day. I get to a scene where I make a fairly simple choice. My mouse then decided to choose that exact moment to promptly spazz the fuck out. It clicked a different option. Now, normally it would be no big deal, just load the last save and continue on. Unfortunately, it happened to be the ONE damn conversation where making that ONE damn choice led to me getting shot with an arrow and killed. In a cutscene. On hardcore difficulty where it wipes your save if you die. /queue never playing the game again since.
My PS3 YLOD'd a few years ago and I had to get a new one. I managed to copy most of my PS3 saves over no problem (save the Assassin Creed games because fuck, apparently they're afraid of save hacking on a damn single player game that I freaking own. I'm not bitter, I swear. It's not like I had 100% on both the first and second one. But no big deal, really). The real issue was my PS1 and PS2 (for the early generation PS3's actually had backwards compatibility, shock and horror) memory cards. Multiple playthroughs of numerous games: Final Fantasies 7-12, Spyro 1-3, Crash 1-4, Jak and Daxter 1-3, Ratchet and Clank 1-3, Resident Evil 4, Guitar Hero 1 & 2, Sly 1-3, and many others all 100% completed and now 100% wiped from my system. Dammit.
Oh..and I had a bag full of Gameboy Advance games get stolen from my parents' car because my dad never locks the door...that sort of qualifies, as the data is saved on the cartridges, to my knowledge.
This is a Fallout new vegas one, I had struggled through dead money and come out on the other side with an actual liking of how it did it's environment and story. I had just finished the last part of the game that I needed to complete. I had everything done, no seriously everything that;s not repeatable (minus the ending) and I saved and quit. When I turned my game on the next day it was nowhere to be found, and sadly I haven't played it since because of it.
Tekken Tag Tournament, I was about 8 or 9 and I unlocked everything (apart from the Tekken Bowl Jukebox thing which you get for scoring 200 on the Bowling game) and all of sudden I had nothing on the game and I got a bit angry and I think I kick my PS2.
Yeah, I was a bit of an angry little bastard when I was younger.