Tom Bombadil says he was in Middle-earth before the first Dark Lord. But the Silmarillion states that Melkor (along with the other Valar) entered Arda when it was first created. This is obviously a contradiction.
I'm not calling Tom a liar. He may have been one of the Ainur who snuck in without associating with anyone. He probably believes he was there first and is mistaken. But at the Council of Elrond he is stated to be unreliable if he were to be given the One Ring for safekeeping. He just doesn't have the mindset to responsibly take care of it. His concerns are limited to his domain. That implies he only sees things from his own point of view, and thus may be considered an unreliable narrator with respect to his origin, which he may have even forgotten.