νῦν δ᾽ ὅτε δὴ καὶ θυμὸν ἑταίρου χώεται αἰνῶς
δείδω μὴ καὶ τεῖχος ὑπερ μόρον ἐξαλαπάξῃ.
And now when he is terribly angry in his heart because of [the death of] his companion
I fear lest the wall [of Troy] will be sacked beyond [i.e., contrary to] fate.
Apollo, the god of prophesy and the one besides Zeus most often associated with seeing into the future, likewise fears that the Trojan walls will come down too soon at Achilles’ hands: μέμβλετο γάρ οἱ τεῖχος ἐϋδμήτοιο πόληος/μὴ Δαναοὶ πέρσειαν ὑπὲρ μόρον ἤματι κείνῳ (“For he was concerned about the wall of the well-built city, lest the Danaans destroy it on that day beyond fate,” Iliad 21.516–517). Zeus’ and Apollo’s fear in these passages is remarkable, and begs questions that anyone who has read the Iliad with undergraduates will be familiar with. If the walls of Troy are destined to fall at a particular moment, how could they fall before that? Is fate something that can be changed? Is Zeus subject to fate or can Zeus alter it?
Ἰλίου ἐκπέρσαι εὖ ναιόμενον πτολίεθρον.
πέρσει δουράτεός [θ’]  ἵππος καὶ μῆτις Ἐπειοῦ.
to sack the well-inhabited citadel of Ilium.
A wooden horse will destroy it and the craftiness [mētis] of Epeios.
These alternative verses make clear that Troy is not going to fall at the hands of Achilles, but rather as a result of the mētis of the wooden horse. Problem solved. But the commentator, in seeking to solve a narratological, mythological, and indeed existential problem, now presents us with a textual one. What is the source of these verses that “some write,” and how do we reconcile them with our received text?
This particular comment derives from the ancient tradition of scholarship known as the “Homeric Questions,” best known from the surviving writings of Porphyry but which, as we can see here, goes back at least as far as Aristotle.  The commentator wonders why Hera required Zeus to swear an oath, when elsewhere in the Iliad (notably in Iliad 1.527) Zeus asserts that whatever he assents to will come true. Underlying the question seems to be anxiety similar to that which we find in the Iliad 20 scholion. Is it possible that what Zeus agrees to won’t come true? Could the story of Herakles have turned out another way if Hera hadn’t made him swear the oath? But the commentator has a solution and goes on to excuse Homer here. He is working with a traditional story, a myth (τὸ μὲν οὖν ὅλον μυθῶδες): “Homer doesn’t invent things nor does he introduce the things that happen, but he recalls the birth of Herakles as these things have been handed down.”
[20.29] νῦν δ᾽ ὅτε δὴ καὶ θυμὸν ἑταίρου χώεται αἰνῶς
[20.30a] οὐ μέντοι μοῖρ’ ἐστὶν ἔτι ζῳοῦ Ἀχιλῆος
[20.30b] Ἰλίου ἐκπέρσαι εὖ ναιόμενον πτολίεθρον.
[20.30c] πέρσει δουράτεός <θ’> ἵππος καὶ μῆτις Ἐπειοῦ.
[20.28] Even before now they would tremble before him when they saw him.
[20.29] And now when he is terribly angry in his heart because of [the death of] his companion…
[20.30a] It is not fated, however, with Achilles still alive
[20.30b] to sack the well-inhabited citadel of Ilium.
[20.30c] A wooden horse will destroy it and the craftiness [mētis] of Epeios.
If we assume an ellipsis here (as sometimes occurs, as at Iliad 1.135–136), we can make it work, but it is more likely that the scholia are quoting from an edition in which the entire passage was substantially different from what we find in the medieval manuscripts of the Iliad.  But these verses are in no way objectionable beyond the fact that they do not survive elsewhere (Edwards 1991 ad loc.).  There is nothing “un-Homeric” about them—they are simply an attested multiform of the verses transmitted by our medieval manuscripts.
Ζεὺς ὅτ᾽ ἀμφὶ Θέτιος ἀγλαός τ᾽ ἔρισαν Ποσειδᾶν γάμῳ,
ἄλοχον εὐειδέ᾽ ἐθέλων ἑκάτερος
ἑὰν ἔμμεν: ἔρως γὰρ ἔχεν.
ἀλλ᾽ οὔ σφιν ἄμβροτοι τέλεσαν εὐνὰν θεῶν πραπίδες,
ἐπεὶ θεσφάτων ἐπάκουσαν: εἶπε δ᾽
εὔβουλος ἐν μέσοισι Θέμις,
οὕνεκεν πεπρωμένον ἦν φέρτερον γόνον ἄνακτα πατρὸς τεκεῖν
ποντίαν θεόν, ὃς κεραυνοῦ τε κρέσσον ἄλλο βέλος
διώξει χερὶ τριόδοντός τ᾽ ἀμαιμακέτου, Δί τε μισγομέναν ἢ Διὸς παρ᾽ ἀδελφεοῖσιν.—‘ἀλλὰ τὰ μὲν παύσατε: βροτέων δὲ λεχέων τυχοῖσα
υἱὸν εἰσιδέτω θανόντ᾽ ἐν πολέμῳ,
χεῖρας Ἄρεΐ τ᾽ ἐναλίγκιον στεροπαῖσί τ᾽ ἀκμὰν ποδῶν.
This the assembly of the Blessed Ones remembered,
When Zeus and glorious Poseidon
Strove to marry Thetis,
Each wishing that she
Should be his beautiful bride.
Love held them in his grip.
But the Gods’ undying wisdom
Would not let the marriage be,
When they gave ear to the oracles. In their midst wise-counseling Themis said
That it was fated for the sea-goddess
To bear for son a prince
Stronger than his father,
Who shall wield in his hand a different weapon
More powerful than the thunderbolt,
Or the monstrous trident,
If she wed Zeus or among the brothers of Zeus.
“Put an end to this. Let her have a mortal wedlock
And see dead in war her son
With hands like the hands of Ares
And feet like the lightning-flashes.” (trans. C. M. Bowra)
Zeus prevents his own overthrow by forcing Thetis to marry a mortal, Peleus, and their son of course turns out to be Achilles. Instead of becoming the supreme ruler of the universe, Achilles will be merely supreme among mortals, and doomed to die young, in battle. This same myth plays an important role in Aeschylus’ tragedy Prometheus Bound. In that play there is a secret that is a crucial part of the plot: Prometheus knows the key to Zeus’ downfall, and it is Thetis. As Slatkin has written (1991:101): “If Themis had not intervened, Thetis would have borne to Zeus or Poseidon the son greater than his father, and the entire chain of succession in heaven would have continued: Achilles would have been not the greatest of heroes, but the ruler of the universe. The price of Zeus’ hegemony is Achilles’ death.”