This game we love so much is a mess, from a data handling standpoint. EA does a lot of funny (and not so funny) things with game data with the result you are describing being quite common. We could begin listing out the various causes of game file corruption but...er, how much time have you got? This might take the rest of the week and it's only Monday on my end.
The NRaas mods that specifically help prevent data corruption and preserve long-time gameplay are Overwatch, ErrorTrap (this is a big one!), Register, Traffic, Traveler if any form of world travel is invloved...we could add a couple of smaller ones in like Dreamer and one that we are not allowed to discuss on this forum for the very specific issues they solve. Where MasterController helps is with its ability to run a Reset Everything from the Town menu on City Hall every so often, usually every once every few sim weeks is sufficient. Are all of these but especially ErrorTrap in play?
There are also player practices that are known to destroy ongoing games, such as allowing worlds to become way overpopulated with residents to the point where the game engine can no longer handle them, trying to load up games from having retreated to the Main Menu instead of quitting all the way to the desktop or Origin (if relevant) in between sessions, scrapbook memory overload, and running the game with too many Dream Managers on inactive households at the same time. We document (or have tried to) all of this and more here:
http://www.nraas.net/community/TIPS-FOR-BETTER-GAME-PERFORMANCE
Testingcheatsenabled is neutral to the game and its data, I don't know where this claim comes from but I have also seen it made elsewhere. I actually play with it on all of the time, this is a feature of Overwatch that can easily be switched on. A couple of the others like MoveObjectsOn and BuyDebug can certainly cause problems of different types if they are left engaged after having been used or while a game is being saved.
