Comment by crazygringo

I can't think of a specific one, but a decent proportion (maybe a quarter?) of HTTP downloads I attempt don't support resuming a partial download, therefore don't support Range. (I.e. resuming always starts from the beginning.)

I would assume this is often because the site in question isn't using Apache etc. to serve a file directly, but is either essentially proxying it to some custom-built file serving service, or a script that processes/authenticates the file in some way, and they just never bothered to implement Range.