After some more tests that all worked, I did have it stop at 12 of 18 downloads on OS X, without debug, and then succeeded with debug.
And then a bunch of other tests all worked fine both with and w/o debug. So I did see the problem. Once. And now I can't replicate it.
I'll look at it some more, maybe tomorrow. But for now the answer for Mac users is to run in debug mode and hope Kovid or somebody more knowledge figures something out.