Accidental, I am sure, I was in a hurry. Just move it back a tad...


I also forgot to move the curves back, since they do not pertain to the final union-ed object anyway.


Anyway, you're right the original geometry isn't quite perfect. But the whole thing would have an easy fix:

Take the other one that did work and rotate a copy of it in place, that way one doesn't have to waste tons of time to fix the broken one.