Louis Letrush (from Riften) in particular is a notoriously bugged NPC. He does that like in the webm sometimes if you give him the horse at the end of his quest, and you bump into him in the forest.
After you do his quest he will randomly start spawning outside of Whiterun's stables as well, and once there are too many of them some of them start spawning halfway inside the ground. All they ever say is "Need something?"
He's considered an essential NPC, so none of his clones can be killed, and he has a little dagger so if you sneak attack him with a bow and he detects you, you can start an eternal war between him and the Whiterun guards who come to your aid
I always found it hilarious.