In the chanting of the holy name of the Supreme Personality of Godhead, this gurv-aparadha is considered the most grievous offense. Guror avajna sruti-sastra-nindanam (Padma Purana)
SB Canto 4
The brāhmaṇas and Vaiṣṇavas are personally glorified by their characteristic powers of tolerance, penance, knowledge and education. By dint of all these spiritual assets, Vaiṣṇavas are more powerful than royalty. It is therefore advised that the princely order not exhibit its material prowess before these two communities and should avoid offending them.
Pṛthu Mahārāja has explained in the previous verse the importance of devotional service for both the rulers and the citizens of the state. Now he explains how one can be steadily fixed in devotional service. Śrī Caitanya Mahāprabhu, while instructing Śrīla Rūpa Gosvāmī, has compared the devotional service of the Lord with a creeper. A creeper has a feeble stem and requires the support of another tree to grow, and while growing, it requires sufficient protection so that it may not be lost. While describing the system of protection for the creeper of devotional service, Śrī Caitanya Mahāprabhu has especially stressed protection from offenses unto the lotus feet of Vaiṣṇavas. Such offenses are called vaiṣṇava-aparādha. Aparādha means "offense." If one commits vaiṣṇava-aparādhas, all of his progress in devotional service will be checked. Even though one is very much advanced in devotional service, if he commits offenses at the feet of a Vaiṣṇava, his advancement is all spoiled. In the śāstras it is found that a very great yogī, Durvāsā Muni, committed a vaiṣṇava-aparādha and thus for one full year had to travel all over the universe, even to Vaikuṇṭhaloka, to defend himself from the offense. At last, even when he approached the Supreme Personality of Godhead in Vaikuṇṭha, he was refused protection. Therefore one should be very careful about committing offenses at the feet of a Vaiṣṇava. The most grievous type of vaiṣṇava-aparādha is called gurv-aparādha, which refers to offenses at the lotus feet of the spiritual master. In the chanting of the holy name of the Supreme Personality of Godhead, this gurv-aparādha is considered the most grievous offense. Guror avajñā śruti-śāstra-nindanam (Padma Purāṇa). Among the ten offenses committed against the chanting of the holy name, the first offenses are disobedience of the spiritual master and blasphemy of the Vedic literature.
The simple definition of Vaiṣṇava is given by Śrī Caitanya Mahāprabhu: a person who immediately reminds one of the Supreme Personality of Godhead, Kṛṣṇa, is a Vaiṣṇava. In this verse, both Vaiṣṇavas and brāhmaṇas are mentioned. A Vaiṣṇava is a learned brāhmaṇa and is therefore designated as brāhmaṇa-vaiṣṇava, brāhmaṇa-paṇḍita or as a Vaiṣṇava and brāhmaṇa. In other words, a Vaiṣṇava is supposed to be a brāhmaṇa already, but a brāhmaṇa may not be a pure Vaiṣṇava. When a person understands his pure identity, brahma jānāti, he immediately becomes a brāhmaṇa. In the brāhmaṇa stage, one's understanding of the Absolute Truth is mainly based on the impersonal view. When a brāhmaṇa, however, rises to the platform of personal understanding of the Supreme Godhead, he becomes a Vaiṣṇava. A Vaiṣṇava is transcendental even to a brāhmaṇa. In the material conception, the position of a brāhmaṇa is the highest in human society, but a Vaiṣṇava is transcendental even to a brāhmaṇa. Both the brāhmaṇa and Vaiṣṇava are spiritually advanced. A brāhmaṇa's qualifications are mentioned in Bhagavad-gītā as truthfulness, mental equanimity, control of the senses, the power of tolerance, simplicity, knowledge of the Absolute Truth, firm faith in the scriptures, and practical application of the brahminical qualities in life. In addition to all these qualifications, when one fully engages in the transcendental loving service of the Lord, he becomes a Vaiṣṇava. Pṛthu Mahārāja warns his citizens who are actually engaged in the devotional service of the Lord to take care against offenses to the brāhmaṇas and Vaiṣṇavas. Offenses at their lotus feet are so destructive that even the descendants of Yadu who were born in the family of Lord Kṛṣṇa were destroyed due to offenses at their feet. The Supreme Personality of Godhead cannot tolerate any offense at the lotus feet of brāhmaṇas and Vaiṣṇavas. Sometimes, due to their powerful positions, princes or government servants neglect the position of brāhmaṇas and Vaiṣṇavas, not knowing that because of their offense they will be ruined.