Occasionally corrupted or bad files can be created during saves that can cause the game to function incorrectly. Custom content or mods being used with the game can also prove to not be compatible with your version or can contain corrupted files that can alter your Sims 3 files completely. To correct this issue you will need to clear out any modifications to the game and your cache and this should resolve your issue. 

To clear your game cache follow the steps below

1. Click on your Hard Drive from your desk top – Usually labeled as Macintosh HD unless you have renamed it.
2. Under the Directory “Places” – Click on the option for your system. Should be the first option with the name of your profile/system.
3. Click on the arrow to the left of Documents to expand the folder
4. Click on the arrow to the left of Electronic Arts to expand the folder
5. Click  on the arrow to the left of The Sims 3 to expand the folder
6. Delete the following files
   • Create-a-Sim Cache: CASPartCache.package
   • Build/Buy mode Cache: compositorCache.package
   • Mod or custom content cache: scriptCache.package
   • New Sims and skin cache: simCompositorCache.package
7. Delete all files within the folder "Thumbnails". Do not delete the "Thumbnails" folder.
8. Test Game. If there are still issues, follow the steps above and complete step 9.
9. Delete all files within the folder "World Caches". Do not delete the "WorldCaches" folder.

This should resolve most Sims 3 issues on the Mac. If errors still persist, clear content and mods per the instructions below.

To remove custom content or mods

1. Follow steps 1 – 5 as listed above.
2. Delete all files within the folder "Mods". Do not delete the "Mods" folder or the "Packages" folder.
3. Delete all files within the "Packages" folder. Do not delete the "Packages" folder.