Error: Subquery returns more than 1 row

Howdy! I just started working with opencart and I have spent some time digging around and can't get this figured out. When I view certain orders in the backend I get the error below - not all of them (and for whatever reason I can edit all orders). Any thoughts on what could be causing this? Thanks in advance for all your help!

Error: Subquery returns more than 1 row
Error No: 1242
select concat(op.model, '-', IFNULL((select a.attachment_code from attachments a, option_value_description ovd, product_option_value pov, order_option oo where a.attachment_type = ovd.name and ovd.option_value_id = pov.option_value_id and oo.product_option_value_id = pov.product_option_value_id and oo.order_product_id = op.order_product_id and ovd.option_id = 16), 1), '-', (select ca.code from color_acronyms ca, option_value_description ovd, product_option_value pov, order_option oo where ca.`Color Description` = ovd.name and ovd.option_value_id = pov.option_value_id and oo.product_option_value_id = pov.product_option_value_id and oo.order_product_id = op.order_product_id and ovd.option_id = 21)) as SKU from order_product op where op.order_product_id = 268

This error means what it says. There is a table that is returning more than one of the same values at a time. For example if it was your seo_url table you could have category_id = 58 twice and it would return that identical error.