This is an edge case, but also really kind of awesome.
Points to a pre-discourse post on BoingBoing. The content of that post is really short, just twenty words which we know Discourse has trouble with, so it’s expected that Show Full Post won’t actually show that post. What’s surprising, however, is the nearly 700-word reply that Discourse does display as the Full Post.
So I dug a bit and it looks like the post page still has the old pre-Discourse comments buried in the HTML, but not displayed. What Discourse shows as the Full Post here isn’t the first comment, nor the first lengthy comment, but a comment nearly 100 comments deep into the thread