SB 6.9.11
hata-putras tatas tvasta
juhavendraya satrave
indra-satro vivardhasva
ma ciram jahi vidvisam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
After Visvarupa was killed, his father, Tvasta, performed ritualistic ceremonies to kill Indra. He offered oblations in the sacrificial fire, saying, “O enemy of Indra, flourish to kill your enemy without delay.”
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There was some defect in Tvasta’s chanting of the mantra because he chanted it long instead of short, and therefore the meaning changed. Tvasta intended to chant the word indra-satro, meaning, “O enemy of Indra.” In this mantra, the word indra is in the possessive case (sasthi), and the word indra-satro is called a tat-purusa compound (tatpurusa-samasa). Unfortunately, instead of chanting the mantra short, Tvasta chanted it long, and its meaning changed from “the enemy of Indra” to “Indra, who is an enemy.” Consequently instead of an enemy of Indra’s, there emerged the body of Vrtrasura, of whom Indra was the enemy.