All glories to Srila Prabhupada!
The topic of aparadha is a very elaborate subject matter.
Naam aparadh is basically offense to the Holy Name and Vaishnav aparadh is basically offence to a devotee.
The offenses against the chanting of the holy name are as follows: (1) To blaspheme the devotees who have dedicated their lives for propagating the holy name of the Lord. (2) To consider the names of demigods like Lord Śiva or Lord Brahmā to be equal to, or independent of, the name of Lord Viṣṇu. (Sometimes the atheistic class of men take it that any demigod is as good as the Supreme Personality of Godhead, Viṣṇu. But one who is a devotee knows that no demigod, however great he may be, is independently as good as the Supreme Personality of Godhead. Therefore, if someone thinks that he can chant “Kālī, Kālī!” or “Durgā, Durgā!”and it is the same as Hare Kṛṣṇa, that is the greatest offense.) (3) To disobey the orders of the spiritual master. (4) To blaspheme the Vedic literature or literature in pursuance of the Vedic version. (5) To consider the glories of chanting Hare Kṛṣṇa to be imagination. (6) To give some interpretation on the holy name of the Lord. (7) To commit sinful activities on the strength of the holy name of the Lord. (It should not be taken that because by chanting the holy name of the Lord one can be freed from all kinds of sinful reaction, one may continue to act sinfully and after that chant Hare Kṛṣṇa to neutralize his sins. Such a dangerous mentality is very offensive and should be avoided.) (8) To consider the chanting of Hare Kṛṣṇa one of the auspicious ritualistic activities offered in the Vedas as fruitive activities (karma-kāṇḍa). (9) To instruct a faithless person about the glories of the holy name. (Anyone can take part in chanting the holy name of the Lord, but in the beginning one should not be instructed about the transcendental potency of the Lord. Those who are too sinful cannot appreciate the transcendental glories of the Lord, and therefore it is better not to instruct them in this matter.) (10) To not have complete faith in the chanting of the holy names and to maintain material attachments, even after understanding so many instructions on this matter.
Every devotee who claims to be a Vaiṣṇava must guard against these offenses in order to quickly achieve the desired success.
Regarding Vaishnava aparadh, Srimad Bhagavatam and Chaitanya Charitamrita are filled with several examples of Vaishnav aparadh and consequences.
Srila Prabhupada has explained this concisely in SB 4.21.37, Purport:
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.
In your service,
Ram Krishna Dasa