When Suruci insulted Dhruva Maharaja, he was just a child. She of course did not know that Dhruva was a great recognized Vaisnava, and so her offense was committed unknowingly
SB Canto 4
The Lord continued: Sometime in the future your brother, Uttama, will go hunting in the forest, and while absorbed in hunting, he will be killed. Your stepmother, Suruci, being maddened upon the death of her son, will go to search him out in the forest, but she will be devoured by a forest fire.
Dhruva Mahārāja came to the forest to search out the Supreme Personality of Godhead with a revenging spirit against his stepmother. His stepmother had insulted Dhruva, who was not an ordinary person, but a great Vaiṣṇava. An offense at the lotus feet of a Vaiṣṇava is the greatest offense in this world. Because of having insulted Dhruva Mahārāja, Suruci would become mad upon the death of her son and would enter a forest fire, and thus her life would be ended. This was specifically mentioned by the Lord to Dhruva because he was determined for revenge against her. From this we should take the lesson that we should never try to insult a Vaiṣṇava. Not only should we not insult a Vaiṣṇava, but we should not insult anyone unnecessarily. When Suruci insulted Dhruva Mahārāja, he was just a child. She of course did not know that Dhruva was a great recognized Vaiṣṇava, and so her offense was committed unknowingly. When one serves a Vaiṣṇava unknowingly, one still gets the good result, and if one unknowingly insults a Vaiṣṇava, one suffers the bad result. A Vaiṣṇava is especially favored by the Supreme Personality of Godhead. Pleasing him or displeasing him directly affects the pleasure and displeasure of the Supreme Lord. Śrīla Viśvanātha Cakravartī Ṭhākura, in his eight stanzas of prayer to the spiritual master, has sung, yasya prasādād bhagavat-prasādaḥ:** by pleasing the spiritual master, who is a pure Vaiṣṇava, one pleases the Personality of Godhead, but if one displeases the spiritual master one does not know where he is going.