There is a kind of existing mythology that lots that have had sims living on them and have then been packaged or moved into the lot bin and shared to another neighborhood are problematic in some way due to the existence of sim references. I've said at various points that I don't think this particular thing is really an issue, and that I don't think the game actually uses those references for anything and that they're just inert data. I haven't discovered otherwise, but I have discovered something that may cause issues if you reuse a lot for multiple families. However, principally, this doesn't have anything to do with sim references, doesn't require the lot to have been packaged or moved into the lot bin, and doesn't seem to be solved by existing lot-cleaning methods.

You know how if you go on a ton of dates using two sims that both live on the lot, and then one of them moves out, then the moved-out sim will show up and drop off a ton of date rewards and flowers? I believe this is related to that, except that it concerns Nightlife mail, although there may be similar issues with date rewards and flowers if they are handled in the same way. Basically, from what I can tell, when you earn Nighlife mail from a date, the actual physical letter that is used in the animations for putting it in the mailbox and retrieving it later is generated off-world, and then its ID is added to an array attached to the mailbox, which is called the "mail bag" in the code and represents what the contents of the mail carrier's bag should be the next time they deliver the mail. As far as I can tell, Nightlife mail is the only thing that's ever put into the mail bag in this way, and bills are handled in some other manner. Anyway, if your sims move out of the house in between the Nightlife mail being put into the mail bag and the mail carrier arriving to deliver the contents of the mail bag, and another family moves into the same house, all of the objects on the lot, including the off-world Nightlife mail letters, are given new IDs and now the ID recorded in the mail bag references some other random object on the lot, which probably won't be able to snap into the mail carrier's hand to do the mail delivery animation, and causes an error when the mail is delivered. There's another array on the mailbox for stuff that's actually in the mailbox waiting for the sim to retrieve it, so presumably if you get Nightlife mail delivered and then move out before retrieving it, the same thing can happen, but it's probably easier to avoid that.

I should note here that alfixes, which famously deletes Nightlife mail, does not fix this issue, since it was intended to fix an issue with the playable sims retrieving Nightlife mail, whereas this error as reported, occurred when the Nightlife mail was delivered by the mail carrier, and alfixes does not modify the functions involved in delivering mail. I've also seen people report that this can happen regardless of whether or not you used the stay things shrub on the lot before the sims moved out, and regardless of whether the lot became uninhabited due to moveout, or because the last resident died.

Ways to avoid this:

• If you're just playtesting a lot before uploading it, just don't go on any dates on the lot. This will prevent Nightlife mail from being generated at all.
• Only move out directly after mail has been delivered and retrieved from the mailbox. This won't help you if the last sim dies at an inopportune moment, though.
• Keep fresh copies of all lots you want to use in the lot bin, and if you want a family to reuse a lot that someone else lived in previously, bulldoze the lot and replace it with a fresh copy.
• Install some kind of mod that prevents Nightlife mail from being generated at all.

As far as I know, the only way to fix the issue after it has occurred is to delete the mailbox and replace it using a hack that makes the regular EA mailbox buyable. Note that if you fail to replace the mailbox, or replace it with a custom mailbox, the lot may not work correctly and you may get errors. I may at some point make a version of the batbox that has a Nuke.../Nightlife Mail option on it that deletes the contents of the mail bag and any Nightlife letters than may be lurking off-world, but I would have to do a little more research to find the GUIDs of the Nightlife letters.

Huh, how interesting! Thanks for sharing kestrellyn. I don't think I've encountered this situation in regular gameplay on a normal lot. However, my university residential lots and dorms often have sims moving out before receiving date rewards (including NL mail). With those, any new student households who move onto the lot tend to be able to check the mail just fine. The love letter (or hate letter) for the previous resident ends up going into the inventory of whatever new sim checks the mail. Is there a difference in the way the game handles move in/out for university subhood lots vs regular lots?

Huh, I'm not sure if there's a difference for campus lots versus regular ones. I wouldn't think so, but I can't think of another explanation for why this problem would happen.

Edit: I just tested this by forcing an error on a sim living on a dorm lot, then moving her out, and then moving her back in and forcing another error. The IDs of all the objects in the two error logs were all different, so I can't see how this error would not occur in that case.

Actually, something is occurring to me now - do you mean the students moved out because you moved them out via the bin in neighborhood view, or because they all graduated? If they graduated, the objects may retain their IDs for some reason, especially if you have mods that change college graduation at all. For example, in my game where I'm using Lamare's YA pregnancy mod, which spawns dialogs asking who you want to take back to the neighborhood with you, and allows you to pick how much money you take, the lot continues to persist on speed three after all the sims have moved out and you manually have to save and quit, so it's possible that that moveout doesn't change object IDs for future residents. I can't remember how it works in an unmodded game anymore. Can you do a test where you force an error on something before the sims graduate, and then move in new sims after the first ones graduate and force another error, and upload the error logs?

I meant when graduating sims move out until the lot is completely empty. For as long as I can remember, furniture and items left behind stay after all students move off a uni lot. Meanwhile in main hoods, moving entire families off a lot whether through the bin or through "ask to move in" always resulted in the furniture getting deleted (unless using something like stay things shrub). I do have cyjon's semester changes mod, not sure if that affects anything.

I'll test your requested scenario in an unmodded game environment and report back with error logs.

Alright @kestrellyn I'm back with logs. I created a clean game folder and moved Erik Swain into a new lot. I spawned the semester tester to have him graduate and made him move out using the phone. Erik moved out after aging up, lot did not reset and all objects including the humble box and a jar of butterflies persisted after move out. Then I moved Jimmy Phoenix into the same lot. Error logs before and after move out are from forcing error on the same object (cheap formica table). Log t259 is pre-move out and t7428 is post move.

Ok, I think that is definitely the reason! I didn't compare every object on the lot, but I compared ~10 different jobdatas, which should be unique, and they all had the same IDs between the two error logs. So it looks like object IDs do not get regenerated in between occupancies if the last occupants all graduated. And when I tested, I specifically tested moving the sim out via the neighborhood screen, and the IDs were all different in that case. So it looks like in this specific case, you would not get the error for that reason.

I don't know of a way to test for the error specifically, to confirm that it does happen under other moveout scenarios, because I don't know how to manually trigger the generation of Nightlife mail, although I guess I could go hunt down the place where it happens and temporarily mod it to be guaranteed, or something.