I was tinkering around in Xcom1Ruleset.rul, making some modifications to weapons as a starting point for figuring out how to mod. (I have a backup of the original, yes). I decided to do things like add auto mode to pistols and heavy lasers, and make auto cannons have auto mode only. Loaded the game up, game crashed. After much trial and error, I found that it was putting the accuracyAuto: and tuAuto: lines into the pistol and heavy laser that was causing it. I opened up the XcomUtil rules file that adds auto to pistol, copied the lines to Xcom1Ruleset, and the game crashes.
I know it's possible using another mod, since obviously the XcomUtil mod does it. I will do such in my own mod, and leave the original Xcom1Ruleset alone. But why is it when I edit Xcom1Ruleset, it causes the game to crash?