Turn off DEP
1. Enable an exception in DEP for the Sims 3 by right-clicking 'Computer' and then left-clicking 'Properties.' Data Execution Prevention is a service that protects your PC from viruses and other attacks. Some programs and games will shut down or display errors when DEP is turned on.
2. Find 'Performance' and examine the DEP settings under the Advanced tab. If it says, 'Turn on DEP for all programs and services except those I select,' Click 'Add.'

3. Browse to the Sims 3 executable. This file is found at 'C:\Program Files\Electronic Arts\The Sims 3\Game\Bin\TS3.exe.' If you're a 64-bit user, look at 'C:\Program Files(x86)\Electronic Arts\The Sims 3\Game\Bin\TS3.exe.'
4. Click 'OK' to confirm the change. If DEP is set to 'Turn on DEP for essential Windows programs and services only,' leave it as is. This setting doesn't affect your game.
Other Possible Fixes
5. Pause the game each time you save. There's no explanation from Electronic Arts as to why this works, but many players have success avoiding Error Code 12 by pressing 'Pause' and then 'Save' or 'Save As.'
6. Check the system requirements for the Sims 3 and its expansions to see if your computer meets the requirements. There's a list of minimum requirements, but the recommended requirements are more important. Just meeting the minimum means the game will run but just barely. Expect to encounter any number of problems, including lag, crashes and error messages. If your PC isn't above the recommendations, upgrade the weak components, if possible.
7. Delete Custom Content from the Sims 3 folder in 'My Documents>Electronic Arts>The Sims 3>Mods' or 'C:\Program Files\Electronic Arts\The Sims 3>GameData>Mods.' Also, if you've downloaded anything from the exchange, start the Sims 3 Launcher and go to Installed Content to remove them. If the custom content is important to you, you'll need to remove a little at a time to isolate the problematic file. Follow the steps in the Resources link to remove a virus-like doll from the game that may also cause this error. This doll has infected many games even if you did not download it directly.
8. Click 'Save As' when you encounter Error Code 12. It won't stop the error, but will usually salvage your game and allow you to save. Type a new name in the box, and try again if it doesn't work the first time.
9. Start the game over from scratch if the error has corrupted your saves. The quick way is to delete the entire Sims 3 folder described in Step 3. The next time you load the game, this folder will be re-created with clean neighborhoods. Alternatively, try to back up an old save as directed by EA Support.