misused words can be fatal...

मन्त्रो हीनः स्वरतो वर्ण्णतो वा मिथ्याप्रयुक्तो न तमर्थमाह।

स वाग्व्ज्रो यजमानम् हिनस्ति यथेन्द्रशत्रुः स्वरतोपराधात्॥

mantro hiinaH swarato varNNato vaa mithyaaprayukto na tamarthamaaha.

sa vaagvjro yajamaanam hinasti yathendrashatruH swaratoparaadhaat..

a manthra which is defective either in swara (pronunciation) or varna (in letter) will give adverse and dangerous effects.. 

The mispronounced word may... become something like vajraayaudha and will annihilate the yajamaana (the one who chants or offers oblations with the mantra).. 

This happened in the case of twastaa who wanted the increase in the number of enemies of Indra, chanted Indrashatroo vardhaswa इन्द्रशत्रू वर्धस्व
 in bahuvreehii instead of tatpurusha samasa..

that is,
 he chanted

indrashcha asau shatruscha इन्द्रः च असौ शत्रुः च (bahuvreehi)... meaning Indra the enemy may grow in valour

instead of
Indrasya shatroo इन्द्रस्य शत्रूः vardhaswa..(tatpurusha)
 that would mean
 Indra's enemies may grow valourous.

Thus instead of getting strong enemies for Indra,
Twsashtaa got a very strong enemy in Indra,, 

So the misused words acted as if thunderbolts which worked against Twashtaa.

This incident is usually narrated to the students of Vedas and Upanishads, during their study sessions.

(Once Vishvaroopa the son of Twashta was appointed as the priest of the Devas with Indra in the lead, because there was some misunderstanding with their usual priest Brihaspati. 

But Visvaroopa had some Asura connection in the mother's side, 

and was not faithful to the Devas.. 

Once, the priest Visvaroopa misappropriated the share of Devas in a yajna and diverted the benefits to the Asuras.. 

Indra got the wind of it and lost no time in beheading Vishvaroopa .. 

Vishvaroopa's father Twashtaa, himself a Deva of sorts, could not forgive Indra for this, 

and therefore he conducted the yajna for obtaining powerful enemies for Indra and to wage a battle against Indra.. 

In fact the demon Vritra was created.. and after a lot of fight and after obtaining Vajrayudha crafted from the backbone of Sage Dadeechi, Indra could finish off Vritrasura.. 

That is the entire story of enmity between Twashta and Indra)