Such was the question someone emailed me. I don't have anything clever to say about it. But let us see it, in Masei (35:5):
I underlined it in red. In printed texts it is an upside-down etnachta. But it took other forms. Here is what Wickes had to say about it. He lists it as one of the mesharsim, that is, conjunctive accents, rather than among the melachim, disjunctive accents. In this he follows the rabbinic writers on trup. Yet he does not think it should be counted as a meshares. He thinks it melody was similar to that of telisha ketana. He also explains the name(s) of this trup symbol. Thus:
According to Wickes-pedia, it occurs once in Torah and a total of 16 times in all of Tanach. According to Wikipedia, it occurs a total of 13 times in Tanach. (Wikipedia also has an interesting homiletic piece there.)
Note that immediately following the yerach ben yomo is another incredibly rare accent, the karnei para. That is, two facing telishas. See Wikipedia on this accent here. And here is what Wickes-pedia has to say about it:
Note that this is a different sort of pazer than the one we are used to.
As to what prompts these rare occurrences, it is so rare that it is not impossible that a midrashic concern prompts it. But I also see, for this one pasuk in Masei, that it is a lengthy pasuk with a particular recurring substructure. I'd have to analyze it and the other fifteen instances, but it could just be the result of taking the system of trup to its logical conclusion, even though it means inventing a new trup symbol and melody (similar to the shalshelet in this respect).