יא צוּרַת שִׁירַת הַאֲזִינוּ (דברים לב,א-מג)--כָּל שִׁטָּה וְשִׁטָּה, יֵשׁ בְּאֶמְצָעָהּ רֵוַח אֶחָד כְּצוּרַת הַפָּרָשָׁה הַסְּתוּמָה, וְנִמְצֵאת כָּל שִׁטָּה חֲלוּקָה לִשְׁתַּיִם; וְכוֹתְבִין אוֹתָהּ בְּשֶׁבַע וְשִׁשִּׁים שִׁטּוֹת. וְאֵלּוּ הֶן הַתֵּבוֹת שֶׁבְּרֹאשׁ כָּל שִׁטָּה וְשִׁטָּה:
However, in most editions of the Rambam, they have him say 70. This misled Cassuto into thinking that the Aleppo Codex was not the Ben Asher Codex which Rambam relied upon. For the Aleppo Codex has 67 lines for Haazinu! (See here and here.)
Minchas Shai takes note of this difficulty. He notes that the author of Or Torah, a masoretic work, notes that in three manuscripts of Rambam it says that there should be 67 lines. (And indeed, as On The Main Line points out, such exists in the manuscript to which Rambam attests that he looked over.) Yet the printed Rambam says 70. And then the question is what to do.
I remember hearing in Revel an explanation, that people used these texts of Mishneh Torah as halachic texts, and so they corrected them to conform to local practice.