This has happened often enough that I'm surprised someone hasn't added a check for it. At the very least, you'd think there should be an interstitial page asking for confirmation if the posted url doesn't match the canonical url.

I thought that referring to your base URL in a canonical tag was a bad idea - why would they do this, since the content on each of the two pages is different?

Interesting. Was this someone at HN updating based on that or does the submission system automatically use canonical over submitted url if present?

Submission system automatically uses canonical