14f056144e
So the error seems to be in cubic_tangent_factor_circle_v3(),
which was introduced with D2001.
I've tweaked the most obvious culprit here - the epsilon factor.
It used to be 10^-7, but I've reduced it down to 10^-5 now,
and it's looking a lot more stable now :)
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BTW, about the derivation of the magic 0.390464 factor I briefly subbed back
as a workaround for this bug, see:
http://www.whizkidtech.redprince.net/bezier/circle/