I ran into a bug that I haven't fully identified, but the results were devastating. I lost more that 200 sims in one event -- The Bomb that Wiped Out Happy Valley.

There was a lot of discussion about it in this thread at Mod The Sims 2. I'll give you the summary here.

Somewhere in the process of installing the first EA patch (released 30 Nov 2004), something caused all but two of the characters in my favorite neighborhood to become unlinked from the game. The two survivors were a married couple who had just moved from one house to another.

Many of the houses in the neighborhood were unplayable; the game would get locked in loop when I tried to enter them. This was especially true of pre-patch houses that I had imported from Strangetown but not yet entered in Happy Valley.

The same phenomenon occured in every household I had played in Strangetown. (I've not played in the Maxis neighborhoods of Pleasantview or Veronaville at all.)

Since I installed the WoohooTeen_6 mod just after installing the EA patch, for a while it looked like WoohooTeen_6 was the culprit. That's why the discussion took place in the MTS2 thread about WoohooTeen_6. However, in subsequent investigation WoohooTeen_6 was exonerated.

I have that mod in my game now and it has caused no problems. It has the great feature of allowing you to invite NPC kids and teens into player households, rescuing them from Never Never Land; and incidentally also allows teens to experience adult interactions. (Except pregnancy. If you want pregnant teens, you need a different mod.)

Now it looks like the smoking gun carries the fingerprints of the 30 Nov 2004 EA patch.

The lesson learned: If you distribute houses, it would be a Really Good Idea to redo any pre-patch households. Just enter them again and save. Then package them up and replace the package that you are distributing with the version you saved with the patched game.

Meanwhile, I have recovered about 75% of Happy Valley in a new version of the neighborhood. The two generations who were born since I first created the neighborhood are lost, but I originally populated most of the neighborhood with houses I had created in Strangetown. I still had those packaged houses so I was able to install them in the new neighborhood and start over from that point.

The other lesson learned: Backups. Make backups! Make backups before you install a patch and then again AFTER you install a patch. I had a backup from before the EA patch, but did not make a new backup immediately after installing the patch, and that is why I lost practically all my sims to one bug.

In other news, I've resurrected the old neighborhood and renamed it Baumdoubt Valley.

I'm using it to test weird things, especially adventures with SimPE.

FYI, I posted a follow-up note in that thread at MTS2 and cross-referenced to this thread at Hullabaloo:
http://www.modthesims2.com/showthread.php?p=161696&posted=1#post161696

So if you got here from there, Welcome!

If you have ideas about how we can investigate this phenomenon further I'd love to hear them!

It looks like I never did post a follow-up in this thread.

I'm convinced that the bomb happened because I installed an occupied lot that I had saved prior to the 30 Nov 2004 game patch, and I had never visited that lot before installing the patch.

This is a repeatable phenomenon. Installing a pre-patch occupied lot triggers the bomb.

I was able to rescue some of the lots by doing a completely clean re-install of the game and visiting those lots before installing Unversity (which includes the 30 Nov 2004 patch).