Look like stepped on the tile with 2 loot pieces picked up one, tried to open inventory and this happened. Situation is very specific but if you wish to do a little more polish, here is the log
Now I see what is going on. It's that secret weapon, that had no name before. It's maybe it that now called a spear or maybe there is a weapon called spear that also cannot be seen on map. You'll get the error when you try to open inventory after looting a spear.
