Not so much a great story as something of hilarity back from early 4th edition. While playing Tau (Prior to their renaming of Tau Empire) I was brought into a large carnage game where each person had with them 500 points a side. There were about twelve of us, and unfortunately I was stuck with Chaos, Tyranids, and the more disordered sort, none of which I was especially fond of at that point. Speaking with the manager, I asked if betrayals were a possibility and managed to back this up with a two minute conversation about lore and motivations behind my army. Something people would never get away with today unfortunately. Anyway, he eventually accepted on the conditions I play my turns in time with the other side and make at least one act of sheer stupidity per turn.

First turn in, no one happened to notice that I had yet to either shoot at anything or move anyone. Then, cries of betrayal suddenly sounded when my Broadside turned and one-shotted a nearby Chaos Lord who had not been stuck with a unit. Then both troops choices I had promptly charged the mass of Genestealer Cultists someone had brought with them. This was supposed to be an act of suicide on my part, only for it to turn into a victory of sorts. Thanks to some poor rolling on both sides, the Fire Warrior's 4+ armour saves actually worked while they merrily clubbed a dozen cultists to death sending them running off of the board. Bare in mind, in the old rules cult mobs tended to eclipse the average group of Guard conscripts in their size.

Were this not good enough, the same thing promptly happened with mauled unit of Ork Boyz, Assault Marines, Hormagaunts and finally a trio of Terminators. By the time these close combat monsters of Fire Warriors were taken down, by a Carnifex no less, they had done more damage than the average mob of Khorne Berserkers.

You sir, are a hero

So I have this particular Chaos Lord who I like to put on a Juggernaut of Khorne, give him an axe of blind fury, and then attach him to a friendly squad of Chaos Spawn.

I swear to Slaanesh, Lord Drake is cursed. Whenever I field him like this, I barrel him at the enemy lines, jumping from cover to cover, and everything goes fine. Then he tries to hit things with his axe, and everything goes to hell. He near-constantly rolls 1s on his demonic weapon test, so he hits himself in the face and goes down to WS 1. Since he's a Chaos Lord, if he sees a character he has to challenge it. He kills the character (usually because he's just a simple squad leader), and when I roll on the Chaos Boon table he turns into a Chaos Spawn. This somehow happens every single time, and I've since refused to use him anymore because it's happened too many times in a row to the point where I'm convinced it's no longer a coincidence. xD

I have a really short story about my tyranids.
It was my first battle (last summer) against my uncle (he showed me and my brother warhammer 40k), and I had deployed 12 hormagaunts, then, besides dem, I had 12 termagaunts, and behind all this, my half-painted tervigon.
He plays necrons, and he had a doomsday arc, a couple destroyers, a necron overlord and a small mob of necron warriors.
Well, there I am, charging directly against him (we didn't use covers) all my hormagaunts got killed instantly by his warriors, then my termagaunts got obliterated by his destroyers (in around 3 turns), and the only unit that didn't die in that game was the tervigon, who took out his necron warrior mob.

The best thing is that my tervigon has never died, it has been really lucky with the regeneration ability.

It certainly wasn't an ideal outcome, but a year ago I was having a 2000pt game with a friend. I was playing Necrons and he was using the Chaos Daemons.

The game went off to a bad start during his wargear rolls. He was using a Daemon Prince and Greater Daemon, while using a horde of lesser troops. However, his Prince and Daemon were heavily upgraded and after their rolls at the beginning of the game, the specific combination of abilities made the two nearly unstoppable. My entire army unloaded on the Greater Daemon and did absolutely nothing to it. And of course on his turn, the two just jumped into my army and began mowing everything down.

However, I was using Zahndrekh and Obyron. Zahndrekh had a Harbinger of Despair with him, allowing him and his squad to teleport across the board. Obyron would then do the same and as he doesn't scatter when within 6 inches of Zahndrekh. I also had a second Harbinger phasing another squad around the table. Through this, I was able to move a sizeable portion of my army across the board, behind the enemy's defences and with the aid of my Doom Scythe, over the course of two turns, I eliminated all of his troops. As the goal of the game was objectives, he effectively had no possible way to hold any of them. Despite him taking several key victory points, I had just enough to edge him out.

THEN... on the last turn he rolled on the warp storm table and got a full squad a troops to appear, which he placed on top of an objective. So a loss. But we put the fear of death in them. It was certainly a very back and fourth encounter.

In our early test games, one of my friends was stomping everyone because he had deathwing terminators. We didn't have answers for them at the time because all we had were tactical squads and assault marines. So his terminator squad is charging my last units, a group of sniper scouts with a rocket launcher. Everyone dies but the rocket launch scout. Then he kills a terminator in melee. All five or six hundred points of army couldn't do it. Rapid fire, charging assault marines. Nope. Rocket launch scout in melee. bonked it on the head.

Back in 4th edition when you could use massacre rolls to consolidate into a new assault I wiped out three ten man tactical squads and one devastator squad in one turn with one full brood of genestealers and a broodlord. It was a good day.

I played a game with my new blood raven army at 1000 points.
the objective was to capture points because that was the only way to score points. No line breaker, first blood, or slay the warlord. After each turn you generated a point from each objective you controlled.
On top of all this only units from your troop choices counted as scoring and special characters don't make things scoring.

I brought Dark angels allied with Blood Ravens
1 company master
2 tac squads plasma gun/hvy bolter
1 BR tac squad flamer/las cannon with a razorback tl/las
and then 10 BR assault marines in a drop pod with one flamer.

my opponent was tau and only brought two troops so my assault marines dropped next to one of the troop squads while my two other tac squads mowed down the other. For the rest of the game he couldn't score anything XD.
I'm really liking assault marines and just got a vanguard box. painted them yesterday, but have no idea how to upload.

reminds me of a similar thing I witnessed with Abbadon the Despoiler getting gunned down by a squad of IG flashlights xD

I love when Tau players in Apocalypse only spam vehicles and then get steamrolled by a bunch of conscripts armed with explosives. Using the "Send in the Next Wave" on a 100 man conscript squad armed with demolition charges and rocket launchers is very humorous as NOTHING can survive that many rolls. It eventually got so ridiculous I was told that I was no longer allowed to use certain TT units because I was "exploiting" the rule sets.