Sometimes a Backbone test receives a 404 (page not found) response code for certain objects, but the page they occurred on did not fail with a 404 error. Other response codes, such as a 12000 (Socket) Receive Timeout, may also indicate a failure to download specific objects, but might not cause the page as a whole to fail.
However, that page could still fail due to other reasons, such as a content match check performed against some of the content that failed to download correctly. If the missing content was an integral part of the page, it may still be rendered in the SCoE as a "page cannot be found" message, just as it would render for an end-user.
Reviewing an Object Breakdown chart for that test run will help you to identify which content, specifically, did not download.