I have many such stories.
Rome; Total War;
My utterly epic Brutii campaign was progressing without incident until just after I had taken Sparta. Some rebels turned up nearby and I decided to send some guys to kill them, because why not? I had no generals there, so the army was just given a random captain. Turned out the rebels consisted of a group of peasants and some archers. We had cavalry. I deigned to fight the battle myself instead of auto-resolve. We swiftly ran them down, losing only 2 men to a thrown horseshoe and a lucky arrow. My captain got promoted to a full general and married into my family. He became one of my best guys.
After conquering the rest of Greece and Macedonia we encountered the first real difficult enemy, Pontus. After several difficult battles I began the invasion of turkey and it all came to a head in this 1 particular siege (Can't remember the towns name now, the one right in the middle of Turkey). I had about 1000 men in 2 armies (1 general, didn't want the AI using the others, so the second force acted as reinforcements), Pontus had at least twice that. I began the battle with catapults and flaming arrows, softening them up and scaring their cavalry. They sent chariots out to get me and having never faced them before I didn't know what to fend them off with. They hit my heavy infantry and immediately tore 1 unit to pieces, but I was able to trap them and only 1 chariot escaped the slaughter (its 2 riders would be the only Pontian survivers). I then began systematically breaking down their walls and moving forces through in multiple places, flanking the enemy as they came to stop me and slowly surrounding them. At the same time I sent my catapults around the edges of the city, chasing down more units.
Eventually my progress began to slow as more and more enemy units were pulled into the battle and I was worried my army was about to break under the numbers, when I suddenly remembered my reinforcemets. They had appeared on the far side of the city and had slowly been walking towards the centre. The enemy units on that side had just left their post to take on my main force, just as my reinforcements met up with my catapults and JUST as a sandstorm rolled in. Under cover of the storm we smashed an entrance and my reinforcements piled into the city. Now I had their entire army flanked and their morale completely broke. Most units routed but had nowhere to run except towards the city centre. All thoughts of tactics broke down as I just had every unit pile into and chase them to the centre. We killed them to the last man whilst the city burned and a storm raged on.
What made my campaign epic later on though, was my plan to ultimately win the game. I was getting close to the end game; the Roman council were giving me harder and harder missions and my popularity with them was dropping, but my popularity with the people was nearly full, meaning the inevitable civil war was about to begin. I began creating armies closer to home so I could move swiftly to take Rome and fight off the Julii, who had a lot of armies close to the motherland. I then bought the only Scipii city in Italy, confining them to Sicily and North Africa. With enough ships being built, my intention was, when the war came, to destroy all their fleets (after strategically positioning mine close to all of theirs) and then blockade every port, leaving them stranded and unable to invade italy, whilst also removing their entire seaborne trade network. Meanwhile, I was in negotiations with the Gauls to convince them to attack the Julii, keeping them busy as I unified italy. Rome was not impressed with my decision to buy the Scipii town though, and gave me orders to return it to them, or be branded a traitor, so I basically had a countdown of when the war was going to begin. Unfortunately I never got to put the plan ino action, but I think it would have worked.
Some other memorable moments; Red Alert;
Playing with a friend many years ago, our mission was to destroy a Soviet base and then infiltrate a bunker. Everything was going fine until we accidentally aggroed the entire Soviet army, which proceeded straight to our base. Naturally we started to fight them off, but there were a lot of them and they slowly pushed closer and closer. Then a V2 rocket launcher fired, straight at our SatCom building. I remember it clearly, the missile seemed to move in slow motion and we realised what was going to happen a moment before it hit. BOOM, the entire building is destroyed in 1 hit, and the map suddenly goes dark. The ENTIRE map, all of the fog of war reappeared in an instant and we were blind beyond the confines of our base walls, as more and more Soviet vehicles came rolling in. The battle lasted another 10 minutes and we had no idea how much more they had to throw at us or what was coming next the entire time. Serious battle for survival.
Another RA one, on a skirmish map, with 7 AI opponents. On a map we had built specifically so we could gain the advantage and everyone else would have poor starting areas, because the way the AI worked in skirmish was to have all the AIs attack each other straight away, then they would ALL attack the player. The poorest of these initial starting locations was an island in the middle of the map, with only a single forded entrance. Unfortunatly, due to an error in setup, WE ended up starting on the island. Laughing at our mistake, we decided to play on anyway, knowing we were doomed. Sure enough, the sounds of battle started from beyond the fog as everyone else started on each other, before suddenly stopping. Then they came for us. 7 enemies, all trying to get over 1 bridge. We had prepared a defence, and a contingency. The defenders did pretty well, and the enemy units had trouble getting over the bridge to the island (in hindsight, it turned out that funnelling 7 players across an entranceway only 2 units wide was actually more advantageous). It didn't last though, and eventually they broke through, but we were prepared, we had built transports and ships to defend them and we started evacuating the base of any and all vehicles and hardware we could. There was only 1 thing we were missing, an MCV. We built 1, but it was too slow getting to the transport and was destroyed. They destroyed our base, but we had survived, and now we existed only as a roving band of battleships and tranports. At this point we were at a loss as to what to do and we considered starting over, but then we saw our salvation; an enemy ore miner. A plan formed. We allied ourselves with this AI and then moved into their base. We then discovered that, if you attack their stuff with a weak enough unit, you can remain allied with them. We had a lowly infantryman start shooting their construction yard and had an engineer on standby. It took a long time, but we got there eventually, taking over their base and all of their stuff, and rebuilding our own. We ultimately won.
Perhaps the funniest moment we had though, was on a mission where the objective was to simply kill every Soviet enemy and building. We had successfully done so after a several hour long match, but the victory message hand't appeared. Obviously, we had missed something. We began scouring the map for a good 10 minutes until we found the single enemy unit that was left. It was an attack dog, hidden behind a tree in the bottom corner. We sent 3 attack choppers to end him.