Popover's arrow is showing an outline (Bootstrap 3)

Question

Using Bootstrap 3.2.0. (And Bootswatch Default, but in my testing this happens without Bootswatch too.)

I'm seeing an outline between the popover and the popover's arrow:

I tried just moving the arrow, but that results in this:

.popover .arrow {
    left: -9px !important;
}

...an overlap, plus and there's also still the thinnest little outline, sometimes visible sometimes not depending on your zoom.

what's the best way to deal with this?