If you don't serve the same content on :80 and :443.

For example, you might be running a Tor relay node which would serve the node directory list on :443 to help people bypass firewalls, but also be using it as a useful and legitimate web server in order to prevent organizations from blocking it wholesale.

Somebody might prefer using http for public stuff (reading blog posts), https for private (editing them).

> In what way are they a broken concept?

Because it's blindly following an inadequate heuristic, and not gracefully degrading if it fails - instead, you get comments like these blaming the author. Even though the author never wanted you to use HTTPS in the first place.

> What justification is there for serving HTTPS in a way users can't expect to consume?

Plenty of technical reasons (eg. what the sibling comments mentioned), but most importantly: there's no Internet/Web standard that I'm aware of which would prohibit me from doing so.

But the author was following convention: serving HTTP on traffic :80, and linking to that explicitly. It's you, the user, who for some reason decided to switch to a different protocol on a different port, and expected it to work even though there was no guarantee or indication that it would.