I think they have checksums every once in a while over their world state, fog of war state etc, and if these checksums don't match it desyncs. Then it creates an out of sync save, probably for just before the desync occured.
Nah, the desync is when two floating point operations do not produce the same outcome, the checksum is detecting when that butterfly has caused a thunderstorm of diverging game states that is measurable. That can happen fairly late , depending on what is hashed.
The strategy of occasional save-game storage and backtracking only works, if the cause is rare and not deterministic.
I'd need to look into it again but I think pretty much everything object state wise gets hashed.
Edit: The checksum for the player includes the content of each attribute of the player, the object state for each object owned by the player, the master object id of that object, the amount of attributes they carry (which is I think resources that villagers carry for example) and the world x/y/z position.
