Visvamitra was a ksatriya, and by undergoing severe austerities he wanted to become a brahmana, but Vasistha would not agree to accept him. In this way there was always disagreement between the two
SB Canto 9
The son of Triśaṅku was Hariścandra. Because of Hariścandra there was a quarrel between Viśvāmitra and Vasiṣṭha, who for many years fought one another, having been transformed into birds.
Viśvāmitra and Vasiṣṭha were always inimical. Formerly, Viśvāmitra was a kṣatriya, and by undergoing severe austerities he wanted to become a brāhmaṇa, but Vasiṣṭha would not agree to accept him. In this way there was always disagreement between the two. Later, however, Vasiṣṭha accepted him because of Viśvāmitra's quality of forgiveness. Once Hariścandra performed a yajña for which Viśvāmitra was the priest, but Viśvāmitra, being angry at Hariścandra, took away all his possessions, claiming them as a contribution of dakṣiṇā. Vasiṣṭha, however, did not like this, and therefore a fight arose between Vasiṣṭha and Viśvāmitra. The fighting became so severe that each of them cursed the other. One of them said, "May you become a bird," and the other said, "May you become a duck." Thus both of them became birds and continued fighting for many years because of Hariścandra. We can see that such a great mystic yogī as Saubhari became a victim of sense gratification, and such great sages as Vasiṣṭha and Viśvāmitra became birds. This is the material world. Ābrahma-bhuvanāl lokāḥ punar āvartino 'rjuna (BG 8.16). Within this material world, or within this universe, however elevated one may be in material qualities, one must suffer the conditions of birth, death, old age and disease (janma-mṛtyu jarā-vyādhi (BG 13.9)). Therefore Kṛṣṇa says that this material world is simply miserable (duḥkhālayam aśāśvatam (BG 8.15)). The Bhāgavatam says, padaṁ padaṁ yad vipadām: (SB 10.14.58) at every step here there is danger. Therefore, because the Kṛṣṇa consciousness movement provides the opportunity for the human being to get out of this material world simply by chanting the Hare Kṛṣṇa mantra, this movement is the greatest benediction in human society.