If you add a category and manufacturer and link a product to them, then delete the category and manufaturer, you can't view the product in Product Management list. I had to delete it using an sql script
You can't delete the category and manufacture that you have a product assigned to. In the product setup screen, first assign the new category and manufacture to the product and save before you delete the old category/manufacture. In the category/manufacture management screens I will add some code to check if a category/manufacture is assigned to a product before it allows delete.