At one point in his demo, he uploads a file but terminates the upload more or less halfway. Then he begins downloading the file - which only progresses to the point it had been uploaded, and subsequently stalls indefinitely. And, finally, he finishes uploading the file (which gracefully resumes) and the file download (which is still running) seamlessly completes.
I found that particularly impressive.
Sometimes there are server-side problems: some dynamic responses (i.e. files that are behind a user account so need the right to access checked before sending) are badly designed so that they uneccesarily break sub-range downloads. This could be seen as a “poor server” issue, but I think it is more a “daft dev/admin” or “bad choice of software” problem.
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[1] admittedly not all, but…
[2] wget and curl do, though not automatically without a wrapper script