There are ten offenses to avoid in chanting the Hare Kṛṣṇa mahā-mantra. The first offense is to blaspheme great personalities who are engaged in distributing the holy name of the Lord. It is said in the śāstra (CC-3:7:11), kṛṣṇa-śakti vinā nahe tāra pravartana: one cannot distribute the holy names of the Hare Kṛṣṇa mahā-mantra unless he is empowered by the Supreme Personality of Godhead. Therefore one should not criticize or blaspheme a devotee who is thus engaged.
Śrī Padma Purāṇa states:
satāṁ nindā nāmnaḥ paramam aparādhaṁ vitanute
yataḥ khyātiṁ yātaṁ katham u sahate tad-vigarhām
To blaspheme the great saintly persons who are engaged in preaching the glories of the Hare Kṛṣṇa mahā-mantra is the worst offense at the lotus feet of the holy name. One should not criticize a preacher of the glories of the Hare Kṛṣṇa mahā-mantra. If one does so, he is an offender. The Nāma-prabhu, who is identical with Kṛṣṇa, will never tolerate such blasphemous activities, even from one who passes as a great devotee.

The second nāmāparādha is described as follows:
śivasya śrī-viṣṇor ya iha guṇa-nāmādi-sakalaṁ
dhiyā bhinnaṁ paśyet sa khalu hari-nāmāhita-karaḥ
In this material world, the holy name of Viṣṇu is all-auspicious. Viṣṇu's name, form, qualities and pastimes are all transcendental, absolute knowledge. Therefore, if one tries to separate the Absolute Personality of Godhead from His holy name or His transcendental form, qualities and pastimes, thinking them to be material, that is offensive. Similarly, to think that the names of demigods such as Lord Śiva are as good as the name of Lord Viṣṇu—or, in other words, to think that Lord Śiva and the other demigods are other forms of God and are therefore equal to Viṣṇu—is also blasphemous. This is the second offense at the lotus feet of the holy name of theLord.

The third offense at the lotus feet of the holy name, which is called guror avajñā, is to consider the spiritual master to be material and therefore to envy his exalted position. The fourth offense (śruti-śāstra-nindanam) is to blaspheme Vedic literatures such as the four Vedas and the Purāṇas. The fifth offense (artha-vādaḥ) is to consider the glories of the holy name to be exaggerations. Similarly, the sixth offense (hari-nāmni kalpanam) is to consider the holy name of the Lord to be imaginary.

The seventh offense is described as follows:
nāmno balād yasya hi pāpa-buddhir
na vidyate tasya yamair hi śuddhiḥ
To think that since the Hare Kṛṣṇa mantra can counteract all sinful reactions one may therefore go on with his sinful activities and at the same time chant the Hare Kṛṣṇa mantra to neutralize them is the greatest offense at the lotus feet of hari-nāma.

The eighth offense is stated thus:
dharma-vrata-tyāga-hutādi-sarva-
śubha-kriyā-sāmyam api pramādaḥ
It is offensive to consider the chanting of the Hare Kṛṣṇa mantra to be a religious ritualistic ceremony. Performing religious ceremonies, following vows and practicing renunciation and sacrifice are all materialistic auspicious activities. The chanting of the Hare Kṛṣṇa mahā-mantra must not be compared to such materialistic religiosity. This is an offense at the lotus feet of the holy name of the Lord.

The ninth offense is described as follows:
aśraddadhāne vimukhe ’py aśṛṇvati
yaś copadeśaḥ śiva-nāmāparādhaḥ
It is an offense to preach the glories of the holy name among persons who have no intelligence or no faith in the subject matter. Such people should be given the chance to hear the chanting of the Hare Kṛṣṇa mantra, but in the beginning they should not be instructed about the glories or the spiritual significance of the holy name. By constant hearing of the holy name, their hearts will be purified, and then they will be able to understand the transcendental position of the holy name.

The tenth offense is as follows:
śrute ’pi nāma-māhātmye yaḥ prīti-rahito naraḥ
ahaṁ-mamādi-paramo nāmni so ’py aparādha-kṛt
If one has heard the glories of the transcendental holy name of the Lord but nevertheless continues in a materialistic concept of life, thinking "I am this body and everything belonging to this body is mine (ahaṁ mameti SB 5.5.8)," and does not show respect and love for the chanting of the Hare Kṛṣṇa mahā-mantra, that is an offense.
(Sri Caitanya Caritamrta--1:8:24--purport).

Śrī Caitanya Mahāprabhu has said in His Śikṣāṣṭaka (3):
tṛṇād api su-nīcena taror iva sahiṣṇunā
amāninā māna-dena kīrtanīyaḥ sadā hariḥ
(CC-1:17:31)
"One should chant the holy name of the Lord in a humble state of mind, thinking oneself lower than the straw in the street. One should be more tolerant than a tree, devoid of all sense of false prestige and ready to offer all respect to others. In such a state of mind one can chant the holy name of the Lord constantly."