The phenomenon that randomly and inexplicably occurs on Northern California freeways causing the inversion of the fast lanes and the slow lanes. At any given time, the fastest lane may suddenly shift from the far left lane to the far right lane, resulting in an extremely counter-intuitive traffic flow.
Friend #1:"Hey man, how did you get back from Tahoe so fast?"
Friend #2:"Well I rode the Nor Cal Inversion on the 80 the whole way back. Also, I wasn't driving a Snubaru."