Tvaṣṭṛ performed a rite to produce a son who would be the slayer of Indra. The word he wanted was a tatpuruṣa — इन्द्रशत्रुः with the accent on the final member, "one whose enemy-ship is directed at Indra". What he actually uttered had the accent on the first member, making it a bahuvrīhi: "one whose enemy is Indra" — that is, one whom Indra would slay.
The son, Vṛtra, was duly killed by Indra. The accent had inverted the compound.