Sarah, 2 things - one is using the ingame world editor corrupts worlds. You are lucky if you can add 4 or 5 tiny lots - because it breaks routing. In CAW when you add lot you can click to rebuild routing when you are all done, so their is no chance a world is corrupted by broken routing. All that said though brings us to the second thing - the particular error code you are getting - Error code 12 - is caused when there is insuffient memory - whether you don't have enough for the size worlds you are playing or if your game is dealing with a corrupted save (corrupted saves can cause memory leak like issues) or the fact you may have a lot of memory eating CC like CC hairs or big memory users. I honestly was not happy with my own game play - which is now practically endless hours in all huge worlds - until I had 8 gigs of Crucial high speed gaming ram and a brand newly built pc with all new everything, made for gaming. But I also have never had an error code of any kind while playing the sims 3 - but I do know what the different error codes mean and 12 is for insufficienct memory to do what ever process you are requiring the game to do - it will up and quit. Windows will just stop working because it hasn't enough memory to do anything else.