We all know that all sorts of nasty errors can be caused by "stacking" items that aren't supposed to be stackable, so I would like to suggest two things that could help to mitigate the issue:
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Modify the "Drop" command so that it will only allow items to be dropped into empty spaces (including valid bins/racks with enough space), and return a "There's no room for that" message instead of moving the object if no valid spaces exist within range
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Prevent shops and NPCs from violating this by dropping things on their own