I have duplicated this in two mod-free saves, so I believe it will happen consistently. If you make the BH premade townie Zain Amri a played Sim, you will see this behavior within a few Sim days.
Zain will be drawn to the town square to play guitar - even if they are the active Sim, and even if you already have other actions queued for them. What's more, each time they do this, they will spawn a new Smooth Strummer 3000 guitar, no matter how many they already have in inventory. I believe the guitar stacking is also happening even as an unplayed Sim, because when they asked to join my household after I had been playing a while and my Sim took them on a date, they joined with 18 guitars already in their inventory. When I immediately play as them in a new save, they do at least start off with an empty inventory but start collecting guitars each time they run off to play.
While in the town square trying to play guitar, even if their needs are low, they will indicate a pathing error if you try to get them to use anything on their home lot (that they just walked away from). They can still use the public bathroom and order food from stalls though. If you teleport them back into the house, then they'll be able to use it again.
Zain's inventory (I did not buy a single guitar, but they've left an additional 3 around the house and one at their business).I woke them up and told them to eat breakfast. They got dressed, grabbed a plate from the fridge, and walked off the lot......to the town square where they vanished the plate of food without eating it (no, it wasn't in inventory either) and whipped out a brand new guitar (the ones in their inventory were untouched) to start playing, despite the low needs.I let them buy & eat candy from the candy stand, then told them to go back and use the toilet. They indicated they couldn't reach it.I'm on Mac, no mods in these saves, and the only expansions I don't have are For Rent, High School, Eco Lifestyle, and Seasons (and Lovestruck is disabled).
Edit to fix photos that scrambled
