I noticed that while playing there are several times when it appears you should be able to walk through terrain, even outdoor terrain in Act 2, or between pillars and pits or other objects in many many interiors, but for some reason your character is blocked.
During one such case I had the idea to switch to Legacy mode, and lo-and-behold there was actually a giant rock in my way.
Video here: Terrain Bug
Not exactly something I’d like to keep running into in any game, but certainly not in HC when I need to know my escape routes.