I had that happen twice. I think something times out in the web interface (haha, I’m just making shit up there), but each time my comment was actually posted.

I’d like to know too. I’ve got comments that got stuck for minutes and then failing to post.

I could be very wrong but for me it seems to be linked to language settings. If my comment won’t send, I select English underneath the text box and then try again and it works.