Also, it's worth noting that even if you have ironclad documentation that a legitimate license had been communicated, that only addresses that you were acting in good faith.  Good faith actions eliminate the possibility for punitive damages, but if you rely on a license declaration that is incorrect (for example because the Flickr user stole it from someone else), you are still potentially liable for actual and/or statutory damages.  Which is why professional publishers usually expect a clearer chain of authorship to avoid getting duped.