The Hadith are checked on several points, say:
The narrators have a faultless string reaching to the time of prophet Muhammad.
The character of the narrators.
The memory of the narrators.
etc, etc
The final and most important checkpoint is whether it is against a verse of Quran.
If it is explicitly against a verse of Quran and it cannot be interpreted to conform to Quran; then it is good for nothing and is discarded. It is surely not from the prophet or our understanding of it is faulty.
Regards
So like the Qur'an the ahadith were passed on orally,as you obviously know the Qur'an wasn't written down in full till he died so maybe the Qur'an is wrong,maybe Uthman edited out the unsavoury parts,surahAn-Najm is said by many to have some heavy editing like the satanic verses.