PLEASE SEE EDIT 2 FOR THE SOLUTION THAT WORKED FOR ME.

I have the Sims 3 base game only.

My system highly exceeds the minimum requirements.

I do have custom content, but I do not think this is the reason for the problem.

The "making a backup of the saves folder, deleting it, and then saving over it" trick doesn't work.

I received the error while I was making a building and I went to save my progress. The way I've built it might've been the reason for this error, oddly enough. I say this because when I remove the lot from the World, the World saves properly after that. I put it back in, and the error is back again. So right now, the only way I can keep this building is just for it to sit in my Bin to rot.

I would like to know if there is any way to fix this because I worked (and still am working) REALLY hard on this building and I would eventually like to share it with others on this site.

EDIT 1: I found a possible solution through google search that I will test out to see if it works. The post is here.

Someone said they made a pond too small, which created this error. While I was working on my building, I tested out the pond portion of the lot, and I went to undo the pond that I made, but I noticed that I still had a bit of an indentation left. I noticed it while I was still working on the building, but I didn't do anything about it. I'll remove ALL of the pond and see if it'll work. I'll update my results.

EDIT 2: It worked! I found the culprit! In the image, it's kind of hard to see the water, but it's there. That must've been what was causing my Error 12 problem. I simply flattened it out and no more error. Must've been a fish spawn bug. Like the poster in the thread I found said, either flatten out the entire pond or make it bigger. I hope this helps others out!