In my third comment to this Stack Overflow question, I put in (one explicit, manually formatted link) and two URLs to other SO questions. Since the amount of characters in a comment is quite limited, and since I had to squeeze three links into that comment, I shortened these links to
http://stackoverflow.com/questions/<ID>. (One of them has a superfluous
/ at the end.)
For reasons I don't understand, markdown failed to turn them into links. Note that markdown did not fail on marking
I have checked both links to be valid by copy-pasting them into my browser's address bar. There's nothing wrong with them.
I have also tried these links in test comments (which I deleted afterwards), and they both work fine there. Another test revealed that the sentence that contains them ("I've tagged http://stackoverflow.com/questions/210616/ and http://stackoverflow.com/questions/2785612 as
c++-faq."), posted as its own comment, will have the URLs turned into links, too.
What could I have done wrong to make this fail?