When that one Argonian tribe learned that humans don’t have cloacas.
Also, learning for the first time not to trust those glowing plants. I don’t watch any previews so I was completely unaware and thought, oh, glowing thing, glowing plants are always good. *thwack!* Made me avoid scuttleblooms for a while (“now what are these shifty plants up to??”) until I learned of the delightful furnishing.
Like a couple the DLCs I've played so far I'm just enjoying how they manage to flesh out a particular race. The Orsinium DLC is another one that brings the Orcs to life.