There is one trope phrase that appears only once in the Torah, in Bamidbar 35:5, a yerach ben yomo followed by a karne farah.
The text of the verse follows; I've bolded the words with these tropes:
וּמַדֹּתֶ֞ם מִח֣וּץ לָעִ֗יר אֶת־פְּאַת־קֵ֣דְמָה אַלְפַּ֪יִם בָּֽאַמָּ֟ה וְאֶת־פְּאַת־נֶגֶב֩ אַלְפַּ֨יִם בָּֽאַמָּ֜ה וְאֶת־פְּאַת־יָ֣ם ׀ אַלְפַּ֣יִם בָּֽאַמָּ֗ה וְאֵ֨ת פְּאַ֥ת צָפ֛וֹן אַלְפַּ֥יִם בָּֽאַמָּ֖ה וְהָעִ֣יר בַּתָּ֑וֶךְ זֶ֚ה יִֽהְיֶ֣ה לָהֶ֔ם מִגְרְשֵׁ֖י הֶֽעָרִֽים׃
And ye shall measure without the city for the east side two thousand cubits, and for the south side two thousand cubits, and for the west side two thousand cubits, and for the north side two thousand cubits, the city being in the midst. This shall be to them the open land about the cities.
For something that appears only once in the Torah, this seems an unusual place for it to show up -- we're in a verse that describes reserved spaces around the Levites' towns in four directions and one of the directions is treated this way? And it's not even the direction itself; it's one of four instances of אַלְפַּיִם בָּאַמָּה in the verse.
What is special about this use of trope?