It is next to impossible that one can concentrate 100 % during recitation of prayers, mantras or any rituals that are conducted. In between our mind trend to wonder and loose concentration, by the time certain stanza of prayers must have recited. Even during recitation of mantras using rosary, we trend to lose concentration and add extra mantras. Even unknowingly we do change certain process of rituals or proper process of conducting ritual are not followed. While reciting in group we failed to catch up with other reciters and in the process leave certain stanzas. All these are errors and generally not acceptable. The good thing is we can purify these slips.
The most powerful way to purify any mistake we have made is to meditate on emptiness. Recite the hundred-syllable mantra and also ring the bell, because the bell signifies wisdom, the wisdom realizing emptiness. The sound of the bell is telling you that everything is empty from its own side, that everything does not exist from its own side. the reason you ring the bell during the Vajrasattva mantra is not as an offering but to remind you to meditate on emptiness. While ringing the bell and reciting the Vajrasattva mantra, remember that the negative karmas and obscurations accumulated through imperfect concentration and various other mistakes you have made are all empty. Remember the emptiness of all the negative karmas and obscurations.
As per HE Zopa Rinpoche one must concentrate deeply on the emptiness of the three circles: subject, action, object. Looking at your negative karmas and obscurations as empty is extremely powerful. One or two seconds of meditation on emptiness is very powerful. That one or two seconds of concentration purifies inconceivable obscurations.
So, whenever you complete recitation of prayers, rituals, counting mantras, reading sutras, always recite hundred-syllable mantra by ringing a Vajra bell holding Dorje at the end and concentrate on emptiness. This ensures that whatever you have performed is made 100% correct. However please note that one should not make intentional mistake thinking at the end by reciting Vajrasattva mantra one can clear it. Such attitude will lead to bad karmic.
ཨོཾ་བཛྲ་སཏྭ་ས་མ་ཡ༔ མ་ནུ་པཱ་ལ་ཡ༔ བཛྲ་སཏྭ་ཏྭེ་ནོ་པ་ཏིཥྛ་དྲྀ་ཌྷོ་མེ་བྷ་ཝ༔ སུ་ཏོ་ཥྱོ་མེ་བྷ་ཝ༔ སུ་པོ་ཥྱོ་མེ་བྷ་ཝ༔ ཨ་ནུ་རཀྟོ་མེ་བྷ་ཝ༔ སརྦ་སིདྡྷི་མྨེ་པྲ་ཡཙྪ༔ སརྦ་ཀརྨ་སུ་ཙ་མེ༔ ཙིཏྟཾ་ཤྲེ་ཡཾཿ ཀུ་རུ་ཧཱུྂ༔ ཧ་ཧ་ཧ་ཧ་ཧོཿ བྷ་ག་ཝཱན༔ སརྦ་ཏ་ཐཱ་ག་ཏ་བཛྲ་མཱ་མེ་མུཉྩ་བཛྲཱི་བྷ་ཝ་མ་ཧཱ་ས་མ་ཡ་སཏྭ་ཨཱཿ
om benza sato samaya | manupalaya | benza sato tenopa tishta dridho mé bhava | sutokhayo mé bhava | supokhayo mé bhava | anurakto mé bhava | sarva siddhi mé prayaccha | sarva karma su tsa mé | tsittam shreyang | kuru hung | ha ha ha ha ho | bhagavan | sarva tatagata benza ma mé muntsa benzi bhava maha samaya sato ah