I was using my knife class and just finished knifing some unsuspecting victim when another guy comes around the corner using his own knife class. We ran around in circles for 20 seconds whiffing with our knives and then my team mate throws a grenade in the middle of it and kills him. I laughed pretty hard. Wasn't a hard fail on my part...maybe moreso on his part, lol.

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Hardcore Team Deathmatch on Afghan. I did a running jump off the cliff to the plane, got shot in the head by a sniper as I threw a grenade mid-jump, the grenade kills 3 of my teammates.

I was once running full speed on Skidrow when a carepackage dropped and killed me randomly. Not really a fail but still funny...

i threw a grenade that bounced off a wall and i blew myself up

Placing a tactical spawn point then getting shanked
camping
having woodlands camo in the snow
threw a Semtex at the wall next to me 
trying to fire a rocket launcher through the airplane window on terminal 
messaging after spawning 
knifing a teammate that I thought was an enemy 
 
the list goes on.

Threw a grenade uphill, just to watch it roll down and blow me the hell up..

I was in Underpass and went into that warehouse thing that has the boilers and such in it. I jumped over the railing into the dark corner next to the boiler, not knowing there was already a guy on the other team sitting in there. I must have stood there nearly 1 minute before he killed me. After I saw that kill cam, I just shut my xbox off.

I'd have to say it was there part where after trying for 20 minutes I still wasn't able to join a single match.

trying to get the nuke and dieing on kill streak 24. it was the closest i've gotten to getting my nuke.

Buying the Stim Pack

when i started and before realizing it was complete noob friendly i thought it was fun. my biggest fail

The only time I did anything that I would describe as an epic failure would be when I was playing Hardcore TDM on Favela, got shot at by some guys in a house, I threw a semtex in the house and by the time it exploded three of my team members had rushed in from a side door and took the dude down and were all three killed by my semtex.  I just left the game :P

Toss the marker for a care package. Right when it drops my teammate spawns, it hits him in the head killing him, the care packages bounces of his head and right into me killing me as well.

its 3 - 3 and im the last man on my team in S&D, only one other player on their team.
he was on top of a small building and i was behind it but knew he was there, so i started teabagging so he'd see it in the kill cam, as you do, went to cook a grenade to kill him with it, and held it for to long whilst everybody on my team was watching

I had a 24 killstreak with the nuke on. I saw a guy, but thought he didn't see me. I scoped in and took my time, but it turns out...he saw me. 
 
 
It would have been my first nuke at the time and well...I was mad.

killed myself and 2 others with a care package. i don't use care packages any more >_>

I can't remember but I'm sure it involved Battlefield 1942, me, a teammate and a vehicle.

Mine was lag related.
 
I saw a dude on radar around a corner, and jumped around to knife him, only, he wasn't actually there, so my knife was just going through the lagged image of him standing there, and I got blasted from behind.
 
When I watched the kill cam, it showed him moving from the corner before I even got there, and then he saw me knifing the crap out of an empty corner, and he blew me away.
 
It was glorious.

Every time I prestige I forget my perks no longer are there to support no fall damage. And I die by falling. Not once, though, I do it many times by mistake. I can't wait until I prestige again and I play on Crash. This is gonna suck.

I think i had a thumb spasm or something and accidentally knifed a team-mate in MW2
 
He thought it was someone behind him and they just started knifing each other (i guess friendly fire was off). This turned into a verbal fight over headsets and the other team ambushed them both 
 
Then the host rage quited

Shot myself while shooting at someone with the riot shield.

" Buying the Stim Pack "

see a guy who i think is afk, walk up to him and teabag beside him a couple times then go for throwing knife and get shot in the head by the guy in front of me and my knife hits the ground

" @REDRUN said:

" Buying the Stim Pack "

XD "

Thought an enemy was an ally.

On Ground War Domination the match was 198-something, and I got an emergency airdrop which was filled with 2 AC-130's, 1 Chopper Gunner and 1 Harrier. I was mad after that.

Playing online.

" Nothing epic but i often blind or stun myself with my own grenades. "

" Playing online. "

The fact I played for over two days and bought the map pack.

My biggest fail online would have to be when I bought FFXI. I was playing for about 3hrs one day and realized I haven't been playing at all but standing next to a flag in the desert trying to get into a party.

My biggest epic fail is thinking that this game's online would be anywhere near as good as 4's was.

Same game. Knife fight. Opponent's care package dropped on my head to my everlasting shame.

Way back near release I was on my way to 10 nukes and I had like 7 or 8 so far. I was playing in a game on Rust and I had gotten all the way up to chopper gunner. My friend decided to join mid-game, and he realized I was destroying his team so as soon as I start getting giddy from seeing the "25 Killstreak Nuke Ready" thing, he quits. Somehow he had host and the game ended while I was still in the chopper.... To be fair I "nuke-blocked" him 2 times after that.

Throwing a frag into the room, thinking it was a sticky, so it takes longer to explode. Ran it. Blew myself and a team mate up...

 I have two common epic fails
 
First, I love throwing Semtex which inadvertantly stick to something i did not see right next to me and kill myself
Second, apparently when standing in trees/shrubs or bushes the Javelin will target the branches and kill yourself :(

" The fact I played for over two days and bought the map pack. "