Proving that either I should be getting paid for this, or I have too much time on my hands: I figured it out! I have attached a 34 player worldbuilder save which works with every civilization in play (Feel free to modify the save to your hearts desire) and for those of you interested in what the problem is a 40 player test save and an explaination below. The problem is caused by the LeaderName strings in the worldbuilder save. The more players in the game the shorter the names have to be. In the 34 civ save I have here the names are still inteligible and distinguishable from one another. In the 40 civ save I've posted, the names seem to be limited to an average of three characters (kind of makes it hard to know who's who. My educated guess is that the leadernames are stored in some sort of shard memory structure which has plenty of space for 18 players but not nearly enough for >34. This is a fairly common programming method to save memory. For the non programmers out there let's say these 11 underlines signify the structure used to store the names in the game _ _ _ _ _ _ _ _ _ _ _ You can put in three names like Bob, Sam, and Jan B o b _ S a m _ J a n two like Billy and Jamie, B i l l y _ J a m i e or even four like, B o _ M o _ J o _ D o and fill up the same space (Although I don't think Do is a name ). It works okay for 2-3 names, but try to fit six in there and you're in trouble. Put too many names in and you'll either overflow your space, or collide the names, either way causing trouble. If you want to see what I'm talking about firsthand load up the 40 civ save I have here. It works but you'll notice that I named all the leaders ABC. Exit the game and load up the 40civTest worldbuilder save into notepad. Find and replace the LeaderName=ABC lines with LeaderName=ABCD. Now load up the file in civilization and it'll crash. From the perspective of fixing it, I'm not sure that will happen. From Firaxis' point of view, the system works fine. 18 players aren't going to run into a problem so the default game doesn't have an issue. I think the easiest solution is like I did in the 34 player save I provided, just shorten the names to something still recognizable. That or only ever play on randomly generated maps.