Robin posted the above some hours ago. As is my usual habit I clicked on the “8 hours 43 minutes” link on the forum contents page which normally takes me to the last post in the thread so annotated.
On this occasion it took me to the Reply-To: form, the one into which I am now typing. This behaviour was reproducible.
My guess is that Robin’s post is the last one on the page and the javascript has a fence-post error which led it to create a new page.