This problem is usually due to a left over Attraction Token. The relationships for a Lot are cleaned when the Lot is packaged and moved to the exchange, sometimes the game messes up and leaves behind Attraction Tokens to Sims in the original neighborhood. Attraction Tokens are just tokens that initialize when your Sim meets someone they are attracted to. So now you have this Sim who is still attracted to a Sim that was in the their original neighborhood, but is not in your neighborhood. The game will try to substitue for this missing Sim with objects, driveways, mailboxes and so on. This causes the game to glitch and one or more of the problems listed above can happen.
This problem also can happen if you delete a Sim or mess around with delete all characters trying to get fresh faces in your neighborhood. Don't do this.
To fix the problem:
Download SimPE. In SimPE click on Tools > Neighborhood > Neighborhood Browser. Let that load. Select the neighborhood your trouble Sim is in. Click to back it up! This is in case you mess up. Then click to open it. Let that load. On the left hand side there is a panel labeled Resource Tree, in it you will find Neighborhood/Memory. Click on it, and let it load.
In the bottom panel there will be a list of Sims on the left hand side listed alphabetically. Scroll to your problem Sim and select him/her. On the right hand side there is a tab labeled Memories, click that. Scroll all the way to the right for the most recent memories. You are looking for one that says something to the affect of [invisible] Token - Attacted to Someone.
Select the most recent one, right click and go to Cascade Delete (to be on the safe side). Then click Commit, on the upper right hand corner of the lower panel. Finally, click File > Save.
If you still have problems then you might have to go back and delete all of the Attraction tokens in their memories. If this is a Sim from a packaged lot from the exchange you will certaintly have to delete all of them. Any erased attractions can be reset once the Sim comes across the hottie again, so no worries.