when removing resource packs from the game models will disappear, sometimes (often) leaving a bugged out space where the piece of stock used to be, by adding a error model that appears when this happens it makes it significantly easier to find and remove these duplicates.
ideally such a error would be Big Red ERROR letters and either show on mouseover (like a widget tool tip) or while in inventory what it is that's missing. the latter might require modifying the item_rolling_stock code to have a default state for missing JSON.

Similarly this system could be used to enable a server with many diverse packs representing different regions, to not require all packs to join without fear of being booted as when you encounter something that you are missing the model of on the off chance you encounter something that you're missing. though you may be greeted with a train of errors.