I just encountered something weird. I loaded my world cell in game to test and discovered that the ground was gone! When I checked it in the CK, I saw that the entire height map had been reset and was completely level and far below any of my world objects. Fortunately, I had backups to restore and only lost about a half hour of work. But I want to know how I did this so I can avoid doing it again, Is there some hotkey I accidentally hit or some file I accidentally deleted? Does anyone know how such a thing is possible in the first place? By the way, I was working on placing lights and LOD generation when this happened. I wasn't fiddling with the height map editor at all.

This can happen if you generate lod (terrain or static) and then start working on the mod. When you save, the the landscape goes haywire. I don't know if this is what happened on this occasion, but always remember to quit the CK after generating lod and then start it again.