https://i.imgur.com/XppQXkY.png
Happened when upgrading 15,000 red belts to blue belts and then a ~thousand or so undergrounds.
My inventory got filled up, but I guess the items that didn't fit were left on the ground.
Sadly I didn't manually save and it autosaved over before I noticed. I had to spend quite some time picking up red belts out of my production zones ;(