The exact same sales order form was sent twice to the office, and was entered twice in error in the same invoicing batch. Normally, once a sales order is posted the first time, if someone tries to enter in another order using the same order number, an error message pops up preventing a duplicate entry. It didn't happen this time. It allowed us to post the exact same order (same line items, same order number, same customer) twice in full.
It was not a partial order or anything of the like.