Tvashtri, his three-headed son and the word said wrong
Indra kills Tvashtri's three-headed son; Tvashtri shuts Indra out of his Soma, and from the dregs thrown in the fire raises Vritra with a curse that comes out the wrong way round.
Vishvarupa has three heads and six eyes. Indra hates him and cuts the heads off; three birds spring out. Tvashtri brings Soma from which Indra is excluded; Indra drinks it uninvited and it harms him. Tvashtri lets the rest flow into the fire with the words 'Grow, having Indra for thy foe'. It becomes Vritra, footless, a serpent, taken in by Danu and Danayu, and grows until it pushes back both oceans. The text says that had Tvashtri said 'foe of Indra', Vritra would have slain Indra.
Vishvarupa is the gods' house priest and the Asuras' sister's son; he promises the share openly to the gods and secretly to the Asuras. Indra, fearing for his sovereignty, strikes off the heads. Tvashtri offers Soma without Indra, saying 'You have slain my son'; Indra drinks it by force. The remains thrown on the fire become Vritra. In 2.4.12 the fire flares and then stays of itself, and both Indra and Tvashtri fear Vritra; in 2.5.2 Indra runs to Prajapati.
What is different here:
Gives Indra a motive (the secret promise). Does not call the curse an error. Adds that Tvashtri himself feared Vritra.
The hymns know a three-headed, seven-rayed son of Tvashtri who owns cattle. Trita Aptya, urged by Indra, fights and kills him; Indra strikes off the three heads and takes the cattle. Tvashtri's revenge is not mentioned.
What is different here:
A cattle raid with Trita as the fighter; no priesthood, no Soma sacrifice, no Vritra born from the fire.
Trishiras, able to take all forms and a sister's son of the Asuras, becomes the gods' house priest to serve the Asuras. Indra learns he was sent by them and cuts off the heads; the Soma-drinking mouth becomes a francolin, the liquor-drinking one a sparrow, the food-eating one a partridge. Divine Speech then calls Indra a Brahmin-slayer.
What is different here:
Told as the frame for RV 10.8-9; ends with the charge of Brahmin-murder and its cleansing by the Waters hymn, not with Tvashtri.
Tvashtri, lord of creatures, makes a three-headed son out of dislike for Indra. The son is a mild ascetic; Indra's dancers cannot distract him, so Indra kills him with the thunderbolt and has a passing woodcutter cut off the heads, promising him the head of the sacrificial animal as his share. Tvashtri, eyes red, pours an offering and creates Vritra, telling him to kill Indra. Vritra swallows Indra, who escapes when the gods create the yawn.
What is different here:
Vishvarupa is innocent and made on purpose; a woodcutter speaks for conscience; no slip of the tongue; the curse is plain.
Vishvarupa, the gods' priest, secretly offers to the demons, his mother's kin; Indra beheads him and the heads become three birds. Tvashta performs a rite to kill Indra, but the chant, wrongly intoned, yields a being of whom Indra is the enemy. Vritra rises from the southern fire and covers the worlds.
What is different here:
Keeps the mis-said chant; Vritra is later shown as a devotee of Vishnu; the thunderbolt of Dadhichi's bones is made by Vishvakarma, not Tvashtri.
Sources: The Satapatha-Brahmana, tr. Julius Eggeling (Sacred Books of the East 12, 41); The Veda of the Black Yajus School (Taittiriya Sanhita), tr. A. B. Keith (1914); Hymns of the Rigveda, tr. R. T. H. Griffith (1896), hymns read for the Vedic layer; The Brhad-devata attributed to Saunaka, ed. and tr. A. A. Macdonell (Harvard Oriental Series 6); The Mahabharata, tr. K. M. Ganguli (1883-96), sections read for the Vedic layer; Srimad-Bhagavatam (Bhaktivedanta Vedabase)