I get this all the time, no matter where I'm at. And although it can happen simply anywhere, it likes to happen the most in Coerthas.

First Players and NPC's stop loading for a while. The music cuts in and out. Then I get hit by a billion and a half invisible walls on my way to a Fate or whatever, sit there in the Fate waiting for the mobs to load so I can actually do something. They never load, I start going back to Dragonhead, then boom, Runtime error.