Not a random encounter per say, but still pretty awesome.
I was about 50 meters from a cave I was supposed to enter. All of a sudden I hear battle music and notice a dragon above me. I got my bow ready and waited for him to attack, but he didn't. Just flew around over me. So I continued my travels. All of a sudden I see the dragon again, diving down behind a hill. I climbed the hill as fast as I could, because well, I wanted to see what the hell was going on. Man, what a fight. 4 bandits, 2 giants, a mammoh, and the dragon was duking it out. I started to laugh like a maniac. My girlfriend wondered what the hell was wrong with me. I showed her the fight and she started to laugh as well.
Anyway. It was pure chaos. The dragon was freezing everyone, the mammoth was fighting the bandits while the giants was going after the dragon. 2 bandits was crushed by the mammoth, and one giant pounded one of the bandit so he skyrockted up into the air. The final bandit tried to flee but got a deadly dose of cold from the dragon. Then the giants and the mammoth turned to the dragon and killed it in just a few seconds. I was thinking at the moment, do Skyrim really need me? Just recruit a few giants, they are excellent dragon killers. Then they all turned to me. The ground was shaking when they started to run at me. I panicked of course. But my trusty companion Sven did not fear. He rushed in, and well, got destroyed. I continued to run to a Whiterun outpost with a bloodraged mammoth and two giants after me. Hoping that they would be able to help me.
But when I got close to the outpost I noticed the guards was already involved in a fight. They had been attacked by bandits, and they didn't really notice me until it was too late for them all. All the bandits and guards was slain by the rampaging giants and mammoth. After the massacer I just kept running witout looking back, all the way to Whiterun. And here I am now. Wondering if it is safe to go out again