The only error I've ever encountered that I couldn't work around was when I added a second terrain after making hours worth of adjustments to the first terrain. Upon saving and reloading, the first terrain would lose all shape, turning into a flat plane.
Plan ahead, add any terrains you might need before you do *anything*.
Back your mod up after every editing session by just duplicating the following folders:
Divinity: Original Sin/Data/Editor/Modname
Divinity: Original Sin/Data/Public/Modname
Divinity: Original Sin/Data/Mods/Modname
