A passage in the latest edition of Plato's Phaedo Footnote ² is slightly but significantly different from what we read in other modern editions.Footnote ³ At 113b–c we read ὁ τέταρτος [sc. ποταμὸς] ἐκπίπτει εἰς τόπον πρῶτον δεινόν τε καὶ ἄγριον, ὡς λέγεται, χρῶμα δ’ ἔχοντα ὅλον οἷον ὁ κυανός, ὃ δὴ ἐπονομάζουσι Στύγιον. But the relative ὅ is undoubtedly wrong, since Plato must be referring the adjective Στύγιον, which never denotes a specific colour, not to the neuter noun χρῶμα but to the masculine noun τόπον. Hence the pronoun needed is the masculine ὅν.

Since ὅ is neither attested in the manuscripts nor explained in the apparatus criticus, I contacted the editor, who kindly confirmed that the reading is a misprint. And while not every misprint in an edition deserves an article, the prominence of this passage in Phaedo and the wide circulation of the OCT perhaps make pointing this one out worthwhile.