I have a Sim who has an issue with his inventory and possible lag as well that no other Sim I've played with has had before.

My Sim's inventory has a problem with Showtime's props- notably the Box of Mystery and the Box of Danger (since he's a magician (and a witch as well)). Every time I start the file with this Sim and check his inventory, there's 2 of each of the boxes. I can delete the extras, but they spawn again when he performs and if I go back to the menu and play again, the bug still happens. I've cleaned out everything in the inventory and it still happens.

What is also notable with my Sim is that wherever he goes, he induces lag and car build up. I have Nraas' Overwatch mod and it always tells me that over 700-1200 cars are removed; in my other games, it's a low number like 20-50, but if I put him into my other game files the number sky rockets and stays that way even if I'm not playing with him. The game tends to lag a bit as well when this happens and it has caused the game to lock up a number of times (not crash, but just stop responding).

I'm at a loss as to what the issue could be as I've tried moving him in and out, resetting, removing the mods but nothing fixed the issues. I'm not sure if it's helpful, but my Sim in particular has work put into him to master every skill if it means anything (not there yet, but a lot of skills though), and had an issue with the lifetime rewards once where the effects stayed but the icons never showed up (I never thought it was an issue so I left it alone).

I'd really like some help with this issue as I really love my Sim and put a lot of effort into getting him where he is and I don't want to delete him and start over again when it could be simply fixed. Any kind of help is appreciated.

Have you tested saving your Sim and his house (or just your Sim) to the library/bin and placing him in town from the bin to see if the issue remains?

Yes I did and it didn't fix anything.

I'll try that, but I doubt it'll fix the issues with the car build up. I also don't think my Sim is corrupted, just the inventory (fine with everything else, goes to work, travels ok, and all that good stuff; it's just the inventory and car build up lag).

EDIT: So I added Nraas Traffic and it lowered the number a little (around 600-700) but it still rose up to the 1000s. Inventory is still bugged with the Boxes.

Your car build up is different from what I've experienced if you moved your Sim to a new town and the build up began almost immediately. In my experience, I used to get the build up when one or more of the non-actives reached the right level in the music career to get the tour bus. I changed the level to 11 using NRaas Retuner and made it impossible for anyone to be assigned that bus. My cars never build up as long as I make that adjustment.

If you use NRaas Master Controller, you can delete the duplicates. That might stick better than selling them off or deleting the usual way. I'm wondering why your Sim is getting duplicates, like the game thinks he has none and needs them. Another thing you can try if it gets out of hand is to have your Sim quit the career, then rejoin and set their level back to the level they were when they quit (using NRaas Master Controller, I'm not sure, but you might need the MC Cheats module also to reset their career level). I'm not currently in a game to check on that.

ETA: Shimrod has 'Careers Rehire at Same Level' mod over on the Sims Asylum. With this in place before a Sim quits their job, they'll be at the same level if they rejoin the career at any time in the future (as long as the mod is in place).

Link to Shimrod's mod (should you or anyone else go that route). Free registration required to view and download.
http://simsasylum.com/tfm/index.php...-at-same-level/

If you decide to have your sim quit and rejoin . MC and the Cheats module is needed:
From City Hall or a computer:
NRaas > MC > Sim > Intermediate > Career >Choose Job to get the job back
NRaas > MC > Sim > Intermediate > Career >Level>Active Sim to assign Sim a job level
NRaas > MC > Sim > Intermediate > Career >Job Performance>Active Sim to reestablish job performance
NRaas > MC > Sim > Intermediate > Career >Bonus>Active Sim Add the simoleon amount to increase his salary to what it was (or more!!)

The car build up is, as far as I remember, caused by the career of your Sim. It's called the "limo bug" by some: it affects all careers that come with a limousine as their carpool vehicle. These limousines keep multiplying during the day and if you also switch households on that day it will get even worse. To stop this there's these mods by nonamena, No Limos!.
The tour bus in the rockstar branch of the music career siffers from the same bug, btw. There's no mod specifically for that, but you can edit nonamena's first No Limo mod to include these as well. Just read what nona actually did to stop the limos from spawning and do the same for the bus with S3PE. I use this edit of her mod together with Overwatch and AwesomeMod and during the last months I've had between 0 and 2 cars cleaned up by Overwatch every night now.

And with regard to your props: I remember that I had this in the early months of Showtime but not anymore. So it could be either Overwatch, ErrorTrap or AwesomeMod that cleans them at loadup. However I also remember that this bug happened mostly while travelling. The simple solution for when this happens is to not travel with props in your inventory. Place them in your Sim's house before you travel.

Edit: Ah, sorry, I just saw that the tour bus is already mentioned. And yes, setting the career level to 11 in the xml does the same and permanently as Retuner does. It's the same what nona did, set every limousine enabled career to a career level of 11.

It happens like I never even moved him anywhere; it's so weird. In some cases, the effect actually stayed in the file when I moved him out, and I had to move out my other sims into a new file to fix their issue (never fixed his though).

I've tried yours and nitromon's suggestions to the car build up and took note of the number of cars that were removed in each notification. It gave me 11 cars, and now it's just stuck to 1 car every so often This is after I got him to quit his job.
After he quit, the boxes disappeared and I made a new save to see if they're truly gone. No duplicates when I went back in thankfully, but I won't know for sure until I get him back at the job and at the same lv. I'll update again when I get him back into the career if the bug is gone or not.

Interesting, but also weirder still since he's actually the only one affected by the bug. I have other sims that use the limo as the carpool and they never had the issue.

Does traveling to Oasis Landing count? I hadn't had my sim go anywhere else but there.

So far I've had it with a Sim in politics, with a fortune teller and all Sims in the rockstar branch. Of course, it's also possible that a mod is making it worse.

And yes, all travelling counts. It must have something to do with the reimport of a Sim when they travel home. Sims that are abroad have no job and when they return home the game reapplies the job. I suppose there's something wrong with recognising the props as being already present and so the game gives them all props again. So, my guess is that any process that temporarily makes a Sim unemployed and then rehires them into their previous career and career level is bound to lead to a duplication of the Showtime performer props.
Do you switch households a lot and if you do so, what method of switching do you use?

That must be it then because I moved a lot to Oasis Landing ^^; hopefully quitting and joining the career fixed it.

I switch households (the usual way) sometimes, but I mostly stay with 1 family per save file. In the save this sim is in, I haven't switched the household.