Persona 4, probably. I didn't even own a PS2 until about a year ago. I bought it used for $30; it came with the hard drive and a controller, but no memory card. I didn't think I needed one, what with the drive and all. I also picked up Persona 3 FES and Persona 4 at EB for $30 and $40 respectively. It wasn't until I got home that I realized I couldn't save without a card, and the nearest EB was nearly an hour's drive away.
I figured I'd just play through as much as I could and leave the PS2 on while I slept. Over the weekend, I spent 40 hours with Persona 4. It was such a gripping game, I rarely paused to break. When Monday rolled around and I could finally drive out to buy a memory card, a snowstorm hit knocking out power to every house within 30km. Progress: lost.
 
It was completely my fault, though, and it let me explore some different early-game choices, so it wasn't that bad.

I guess I'll classify this as "power going out" 
I had a really boring day so decided to play Bomberman Land Touch! on DS. Well it turned out that I kept playing all day and ended up just a few games before the last boss. It was time for me to eat so I closed my DS lid. After eating I came back and muscle memory took over and I flicked the power switch.....and no save for the whole day and all my progess went out the window :(

Never really lost progress from power outage, it rarely happens here.... From losing saves via corrupted data sure. 
 
I was 3/4 into FFVIII and I saved in a memory slot that always corrupts.  I played through it again and accidently did it in the EXACT same save slot at the exact same part of the game that is around 3/4.  
 
 Needless to say losing almost my entire game twice... I never beat FFVIII but damn that was a fun game.

I've never had that problem. Lucky I guess 

This, thankfully, has never happened to me

i don't think that has happened to me before. i would say on the NES but NES barely had any saves.

Lost 80% of my game progress on a game because the memory card was saving when the power went out and the data was corrupted.  Happened to my Guitar Hero 3 game too.

Well, none I save my work like a stopwatch, why not my play?

The worst that happened to me was that I was on the final boss in the original Dragon Warrior, power goes out, I come back and the save file is gone.....yup never played it again. A friend had about 30 some hour lost on ff7 when his brother overwrite his save file (yeah I know it doesn't have anything to do with the power going off).

my cousin did a runthrough of metal gear solid 1, by the last 'fight' my dad tripped over the ps power cable and he lost it aall

 I have only ever had power outs happen while I play games online. Which in some ways is even more annoying because I normally only play friend games, so I hate knowing that they are stopping the game and waiting for 10-15 min to try and figure out what happened to me and why I just quit, and I can't do anything about it.
 
As for lost saves, I played the original Mass Effect on my 360, but I played 2 on my PC so I had to play 2 without being able to load my first game. So after I played 2 I bought 1 off Steam for like $5 during one sale and powered through that and loaded it into 2 and was going to do the same so that I had a full save ready for 3, but I got bored of playing 2 so soon after my first time around that I stopped before I could make my new Master file. I have since lost all of said save files anyway, so when 3 is a few weeks away I plan to marathon 1 and 2 in preparation for 3 and just use that as my main save.

Dead Rising 2. got soooo fucking far but I never saved and my PS3 froze. piss.

I have painful memories of the days when gameboys ran on batteries and couldn't be charged, and games couldn't be saved. HOURS OF SUPER MARIO, LOST FOREVER.
 
My old big grey gameboy still has bite-marks on the screen from an unfortunate incident along those lines... yeah, I was prone to fits of rage as a child. Oh wait, I still am. Ahem.

Back In Operation Flashpoint, there was no Join In Progress in multiplayer games. Operations could take very long on the huge maps, and if you fell out, you could be stuck outside for hours, waiting for the clan to complete or fail the ongoing operation.
Luckily the sequels(Arma 1 and 2) has JIP so its no big deal if you fall out.
 
We rarely get any unplanned power outages in my area, despite massive amounts of snow in the winter.

i actually legitimately got that achievement :(

Interestingly enough the power went out while I was playing Turok  for the 360 and corrupted the 3 hours worth of saves. Since that game was such a giant piece-of-shit though I consider it a blessing and I've never touched that game since. 
One real bummer though was one of the missions in Brood War. That one lost me an hour or so of just absolute frustration and bullshit that I had to do all over again.

Probably a few hours on Mass effect 1. Most of the time if I am forced to replay stuff I just stop playing the game and never come back to it.

My sister saved over my Kingdom hearts save. I had about 100ish hours. 

I played Final Fantasy VIII for about 10-12 hours on a saturday. Then my parents asked me to go out to eat with them and they were leaving at that moment in which there was no save spot around. I freaked for a bit and decided to leave my console on. Hours later when we returned, I came back to my PS1 perfectly fine so I started playing it some more. Got to the save and decided to quit but wanted to play some original Tony Hawk. Played for about 20 minutes and no shit my memory card slot 1 made the weirdest noise and started smoking very faintly. Console still ran but wouldn't read memory cards and my memory card lost all its saves, including a pretty lengthy 60 hours into FFVIII (end of disc 3) tony hawk, castlevania SOTN, and others i wish I still had to this day.

When my Xbox 360 broke I rang up GAME to see if they could fix it for me, the manager said to clear my hard drive and he'll upgrade my hard drive for free, when I got there he said he couldn't replace the hard drive or the Xbox 360 so of course I completely raged. Anyway I had to send it to Microsoft to get it replaced where they charged me £70 or so, turned out I didn't need it repairing as one of my friends could of got me a replacement for free so I had to get it back from Microsoft and get my money back which took 3 months. Absolute scumbags. 

I lost many ours of gameplay multiple times on pokemon during my childhood from my gameboy running out of power.

" @Resident4t said:

" I got to Disc 2 on Metal Gear Solid The Twin Snakes(forgetting to save) when the power went out leaving me back to Disc one. So....That's like 30 minutes of gameplay? :D! "

Twin Snakes had two discs? What the hell, it's not as tho augh Metal Gear Solid is that huge a game, right? "

I remember playing Oblivion for four hours straight without saving only to have the power to go out. Wasn't really a big deal for me since that is a hella good game.

" I got to Disc 2 on Metal Gear Solid The Twin Snakes(forgetting to save) when the power went out leaving me back to Disc one. So....That's like 30 minutes of gameplay? :D! "

Not exactly a power outage, but a corrupt save in FE4 forced me to restart. For perspective, I had reached Chapter 7.

I was trying to clear Silent Hill 3 without saving.  The power went out right as I entered the Church, which is the last area of the game, and, seeing as I had not saved, well, it really, really sucked..

Only on the NES... on the PC I'm an "obsessive saver" but I have lost my Dungeon Siege 2 save for my mage character... 250 hours gone...

I've been saving in games every 30 seconds or so (the ones that let me, ex. oblivion) 
ever since I lost about 30 levels on all my party pokemon when I was 12 (I never touched that game again)

I had just beat Banjo Kazooie: Nuts and Bolts, I went to eat. I got back, I booted the game up and managed to start a new game and overwrite my previous save..... Doesn't count as "power going out", but I will NEVER forget and can never forgive myself for my usual "doing-something-dumb-while-thinking-about-something-completly-different".

all of my progress in rise's dungeon in persona 4. and i was fighting the boss too.

I lost all my RPG saves on my mega memory card as the worst, but that wasn't the result of a power outage.
 
Once I spent three hours making a snowboard in Cool Boarders 2 only to have my fatass friend trip over the power cord... which is totally worse BTW. Games are pretty linear, so getting to exactly the same point is much easier. Art, on the other hand, cannot be recreated as easily. I was livid.

Probably a game of Civilization IV. I spent a good few hours in before the power went out.

Two words...RESIDENT EVIL.
 
Back in the day I'd try to run through all the games without saving, managed to do it too save for RE3. God Damn Nemesis.

I dont't have any experiences of that happening but my friend was playing Ratchet and Clank crack in time and he was saving and bam power goes of and every saved game disappears.

Some people actually play that obsessively.  I don't see myself playing Katamari that much, but I could with a game I enjoyed.

Fable 2, all of it. Playing Fable 2, surge hit and Xbox Red Rings. For some reason, all of my memory from Fable 2 just didn't work after that. Eh

" Mass Effect 1. All of it. "

When I first bought a PS2 that was all I bought. When I eventually got a game, it was Vice City. I didn't have a memory card. I played the game with all my free time after school every day, since it was my first game for my first console. On the 5th day my PS2 fell over and it froze.

Mass Effect 1. All of it.

This has yet to happen to me. Usually in a thunderstorm I'll shutdown the Xbox. Haven't
had any rolling brown outs in awhile & I don't usually leave my Xbox running
when not in use.  We seem to have rolling brownouts at work more often than my home,
it's located in a more densely populated area.

" This has to be one of my main fears when it comes to gaming so I have an OCD attitude when it comes to saving. Sometimes, I'll save twice in the span of a minute just because I'm not 100% sure I actually went through with the save the first time.
 
My worst amount of game progress lost in general is 40 hours in Dragon Age for the PC. I was in the Dwarf caves last I had played it. The very next day, my computer refuses to turn on. Turns out my hard drive was banged around a little too much from a 1 inch drop between where it sits and the carpeted floor. I found the game fun and was willing to go back but I lost the instruction book and the code to install it along with it. "

This has to be one of my main fears when it comes to gaming so I have an OCD attitude when it comes to saving. Sometimes, I'll save twice in the span of a minute just because I'm not 100% sure I actually went through with the save the first time.
 
My worst amount of game progress lost in general is 40 hours in Dragon Age for the PC. I was in the Dwarf caves last I had played it. The very next day, my computer refuses to turn on. Turns out my hard drive was banged around a little too much from a 1 inch drop between where it sits and the carpeted floor. I found the game fun and was willing to go back but I lost the instruction book and the code to install it along with it.

lost my ps1 memory card so I was playing metal gear solid through in one go and the powere went out at the part where You heat/freeze the key card to "deactivate" the metal gear

I got to Disc 2 on Metal Gear Solid The Twin Snakes(forgetting to save) when the power went out leaving me back to Disc one. So....That's like 30 minutes of gameplay? :D!

So in Beautiful Katamari there is a 40 achievement for playing the game for 50 hours of playtime and an 80 point achievement for 100 hours. Let's just be straight: there is no way in hell anybody would ever actually reach 20 hours (let alone 50 or 100) in that game naturally. 
 
Yeah, so I've just been leaving my 360 on all day and while I sleep and stuff. I already got the 50 point achievement and saved after I got it. Then, if I did the math right in my head, I was at somewhere like 99 hours and 45 minutes-ish, but then the power in my entire neighborhood just goes out for no reason. So I lost all of that...greeeeat. 
 
There's been some other instances where that happened to me. Like there was that extremely hard mini-game level in the first Super Mario Galaxy where you have to hurl bombs at the trash and right when I got the Star the power went out.