Any idea how to reproduce that?

There might be some errors where you can quite literally walk through any wall, it seems to happen to people walking to revenants a lot. It mostly happens when loading I belive. I have it on video in my quest guide.

youtu.be/nWpIXDraOJc?t=4m26s

The 'loading please wait' seems to be involved with it at least.

May have a possible fix for that problem. Difficult to really verify, because I'm unable to reliably reproduce the problem. Let me know if it happens again by just posting another bug report.