When Peter quotes from the prophet Joel at the end of his Pentecost sermon, he seems to begin the quotation with the words, “And it will be in the last days, says the LORD, I will pour out my Spirit (ἔσται ἐν ταῖς ἐσχάταις ἡμέραις λέγει ὁ θεός ἐκχεῶ ἀπὸ τοῦ πνεύματός μου)” (Acts 2:17). This quotation comes from Joel 2:28ff., but here’s the odd thing: no version of this verse that I can find includes the phrase, “ἐν ταῖς ἐσχάταις ἡμέραις.” Instead, there’s unanimity among the ancient versions I can consult in favor of the prophecy beginning with, “afterward”: “אַֽחֲרֵי־כֵן,” in the MT; “μετὰ ταῦτα” in the Rahlfs-Hanhart LXX (w/ no comment re: divergences in the manuscripts); and even “post haec” in the Vulgate, despite Jerome’s knowledge of the altered prophecy in Acts.
This doesn’t seem like a niggling question to me, since the reference to “last days” warrants Peter’s appeal to this verse as an event that belongs to the messianic age, possible only after Jesus has sat down with glory at the Father’s right hand (cf. Acts 2:33). So, how did Acts’s version of Joel acquire this phrase? One possibility (which might be further illuminated by a more exhaustive survey of Old Greek mss., the sort of thing you’d get in the Göttingen LXX) is that the version of Joel that Peter/Luke knew in fact did contain the phrase, but that still doesn’t resolve the fundamental question, namely, why would someone (i.e., the anonymous scribe) have thought to add it there?
Here’s a tentative suggestion. In Hebrew, the phrase “בְּאַחֲרִית הַיָּמִים” (“in the end of days”, cf. Isa 2:2, Micah 4:1) sounds a lot like “אַֽחֲרֵי־כֵן” (afterwards): both employ the root “acher,” “after.” Moreover, Joel mentions that the wonders he details presage the “Day of the Lord,” which throughout the Prophets at least evokes the end of the present order, even if it doesn’t quite denote it (cf. Isa 13:6, Amos 5:20, i.a.). And finally, it’s striking that the phrase “בְּאַחֲרִית הַיָּמִים” appears twice in the MT, in both cases describing the gathering of the nations to Zion to submit to Israel’s God and to live in peace with one another (Isa 2:2-4, Micah 4:1-5).
This third detail is most significant to me. Perhaps Peter saw that the gifts of the Spirit promised in Joel were being poured out to knit together Jews and God-fearers from all nations, reversing the conditions of Babel that estrange people from people — perhaps he saw this, and realized that Micah and Joel had really been prophesying about the same thing. Or perhaps the Spirit himself, through a divinely ordained slip of Peter’s tongue, is communicating that to all of us.