Some times walls can disappear before it went out of viewport. I seen it a number of times from 1.8-pre1 up to current 1.8-pre3. I have FOV=90 deg. See screenshots below and its orientation angles. Walls disappear/disappear with respect to north/west/south/east orientation. I can see the world thru disappeared walls.

It's difficult to reproduce. Currently last time I catch the bug I exit from the world and load it back. Bug is actual again. Then I made screenshots and force a crach with F3+C. Crash report and latest screenshots are attached.