What I've found for Object references is that usually something was misspelled somewhere somehow
I don't make that type of mistake (it's my superpower!) As far as I can tell, I had no such mistakes in the ~14k lines of XML I wrote. I had to deliberately introduce one to see what would happen cause I thought they were just being ignored or something.
In all seriousness though, in the event of a bad ItemID - regarding shop stocks, anyway - it will crash on load. The game appears to populate the items friendly civs would offer if you send a merchant to visit the friendly civ as soon as you discover a civilization. That happens on world creation for the first Merchant City-State. Thus, I'm reasonably confident that this crash isn't due to a bad ItemID. Still working on testing it... getting a new world up to being able to host merchants takes like 3 hours. =|
Wish I had a faster way to test it. x4 or x8 game speed buttons, RoboB0b? Or the ability to mod friendly/goblin civilizations' distance? Cause 1 hour for the ambassador and then 1 hour for a merchant wouldn't be too bad.