It is possible to change barcode on a checked out item. This will cause problems when handling the item later. Suggestion is to add an option (syspref?) for protecting certain fields in the edit item form. There might be other fields that other libraries want to protect from altering on checked out items. Normally we don't edit checked out items but when you have many items on one bib, it's easy to make a mistake and end up on the wrong item.