This was the craziest surprise battle I've had yet.
I fast traveled to a town and arrived as a dragon attack was in progress. My companion and I started attacking the dragon and had it worked down to about 30% health when it flew up and landed in a spot on the edge of town, right next to a giant. The giant aggro'd and it and the dragon started to fight each other. I shot an arrow at the dragon which hit, but I guess that was still enough to redirect the giant's aggro toward us. So now my companion was circling the giant while I was trying to hit it with arrows and simultaneously keep track of where the dragon was at. My companion suddenly disappeared, either knocked down or stuck on the terrain. Next thing I knew the giant was barreling toward me as I continued to fire arrows at it while backpedaling. There were like 5 arrow shafts sticking out of its head and upper body and it still kept coming. All of a sudden I started taking damage, even though the giant wasn't close enough yet to hit me and the dragon was flying off in the distance. I turned around, expecting to see a second giant or something equally terrible that was about to kill me, and to my absolute surprise it was a little mud crab, snapping away at my feet. The idea that this little crab would go full on Leeroy Jenkins and rush into the battle made me laugh so hard I had to pause the game. Once Leeroy the crab was dead I finally brought down the giant, found where my companion had gotten stuck at, and we finished off the dragon pretty quickly. The way the battle happened with no forewarning or time to prepare made it more fun. This game has been so good, from the very start.