@CharlesStaats Right, the latest upload of hobby.dtx should fix this. But it introduced a concept I'm not sure about. Basically, we have the possibility that a coordinate can be used as the end of one Hobby path and the start of the next (the tangent key forces this, for example) so its options might relate to the first or the second path.

So I need a way of distinguishing and I've built that in. But I'm not convinced that I've made the right choices: it ought to be that if only one path is involved then the obvious path is always chosen, but that might not be the case at the moment. For example, if you type \draw ([closed]0,0) .. (1,1) .. (2,0); then when the first coordinate is read, TeX doesn't yet know that it will be constructing a Hobby path so the closed key technically applies to the next path.

(I'm sort of thinking aloud here, not expecting anyone to respond - but if anyone does have a good idea then I'd be delighted.)