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