I'm not quite sure because it has happened with both the gravel and the stone, but eventually after I use up a couple items and keep playing, I get an error message in my browser if I pick something up, don't know what really triggers, maybe an out of bounds error in an array, usually happened when I had at least more than 1 slot available. Perhaps it could also be an item trying to assign itself to multiple slots. ยฏ\_(ใ)_/ยฏ