It happened to me the other day while I had a link to a book on amazon.com that I had referred to in my comment. My attempts were not actually being posted. As soon as I removed the link, it posted fine. Also the whole comment was a link, which may be relevant.
That used to happen all the time if you were spoofing the header info (Referer), and indeed the new post would go through regardless of the warning. It has stopped, but maybe it is a combination of this with something else.
[–] AlbinoStoic ago
Happens repeatedly to me as well.
[–] AlbinoStoic ago
100% error rate when comment contains a bitly link