Thanks for providing the 2000 version; I only have Access 2003 here at work (2010 at home).
The main thing I noticed is that you have the quantity field in the order_products table as text, it shoud be a number (long number if you do not expect fractional quantitites, otherwise single or double precision). After I changed the data type of the quantity field to a number, I deleted the subform and rebuilt it. It is working in the attached database.
Thank you, I'll follow your advice as I get to work tomorrow. Honestly, though, I think this is caused by an access bug or something, because I set that field as Text since that's what you had used for your example CustomerOrders.mdb.