I just noticed in my Sims game that the game is creating musical instruments in the Sim's inventory. I believe this behavior started with the City Living patch or EP. Creating large numbers of instruments in multiple Sims' inventories is likely a source of lag in the game and/or slower loading times.
As far as I can tell, it seems to be linked to the new ability for a Sim to play an instrument at the San Myshuno outdoor venues. I have several played Sims who have high music skills and who can be seen playing fairly regularly when I visit those venues while playing other Sims. (I have multiple households that I play rotationally.)
So to break it down: I go visit one of the festival areas with Sim A. While there, I see Sim B (also one of my played Sims, but from a different household) playing the guitar. Later (after a fair amount of play), I rotate to play the household of Sim B. I look in the personal inventory of Sim B and find 16 guitars instead of just one! (16 is just an arbitrary example, though I did see some large numbers. 22 of one guitar, I think. I expect it may depend on how much play time passes with visits to said lots.)
It's not a bad way to make a few extra Simoleons, selling off those extra instruments (especially my violinist, who got copies of the 15k top-end violin!) but as I said, I expect this is contributing to the lag and long load times in the game, and could cause problems down the road.
Edit: Just found out this issue has been reported already. I didn't find it in my own search as I was searching for 'instruments' or 'musical instruments' rather than specifically 'guitars.' (I do know that it happens with violins too.) Please feel free to merge if needed. The original report is here: http://answers.ea.com/t5/The-Sims-4-Bug-Reports/OPEN-Multiple-Guitars-added-to-NPC-inventories/m-p/5680337#M29648
