Sutra:
17. sollum kramamozhiyas sonnAlum than SvarUpam keda nillAthu.

Meaning:
Even if one were to slip on the proper method of reciting it (with faith and completely surrendered), the Thirumanthra will not slip from its nature, which is providing full protection to those reciting it.

Sri PBA Swami's Sarartha Deepikai:
This is another way in which the Thirumanthra's greatness is being celebrated.

Previously it was stated that there are countless manthras that speak of the Lord. Any manthra will work only for those who fully believe in it, utter it with great bhakti and sraddha and in the proper manner. But the Thirumanthra will protect everyone who recite it, no matter how they do it. That is, it never fails in its nature.