A trailing period in URLs in written prose is always ambiguous. Is it the end of the sentence, or part of the URL? If a trailing period is legal, what do you do if a URL is at the end of the sentence? Leave a space between them? It's fine for HTML because you control where the a tag goes, but auto-linking like HN does can't distinguish between the two cases.
HTTP and e.g. markdown has solved this with angle brackets. I tried that (didn't work), and putting a question mark at the end (also didn't work).
What did work was putting two full-stops, but that was so ambiguous that I left it as-was. Wikipedia has a redirect in place so obviously HN isn't the only one with problems.
