I've never had an error 12 saving this town, so I thought I might share my saving process, see if it could help anyone. When I save my game, I have all my Sims on one lot, I pause the game, and I always choose 'Save As' then give each save it's own name by adding one to the number, such as Dynasty House 357, or Ratty House 12 or Challenge 5. Then the next time I load in, I delete all but the important saves, the three most recent, and any important backup saves. I hope maybe this might help someone save more succesfully, nothing is more frustrating than playing an enjoyable session of Sims, and the having it go *poof* up in smoke.
Once again, I am love Union Cove! Though there does seem to be a few too many Sims around, they've been moving out really quickly, I wish they wouldn't.