It happened (for this patch) because You had a Specific Sword customer, they specified what grip, guard, and blade they wanted, and you didn't provide that.
Don't mess up a Specific Sword Sale, or, don't be holding an incorrect item while the specific sword customer is still there.
Just Dismiss them to begin with and don't serve them.
Don't serve Shield Customers either, as they will cause multiple customers. - just Dismiss them immediately (hopefully both are already addressed, as it sounds)