I'm puzzled by this, too. But I think Willison (the linked tweeter) is complaining about a web-specific manifestation of this, when a web page has a sub-section that needs to scroll independently of the larger page that it's contained within. (The list of messages in Gmail is a good example of when this is useful.) Having said that, though, I'm not actually sure under what conditions his lack of scrollbar is occurring. Personally, I've noticed this can be more of an issue on iOS devices, where sometimes the contents of a <div> are meant to be scrolled, but it's almost entirely nondiscoverable due to iOS's lack of scrollbars.