Several people have mentioned that they've received errors when posting comments, and some people have re-posted the comment later. I'm not sure what's up with the errors—it would be great if anyone who experiences them could drop me a note in email with the URL of the error page and the text of the error, and any other info that might be useful about what you were doing when it happened. But I think the error is happening after the comment is successfully posted.
So if you get an error, try reloading the relevant entry page or comment page in a new browser window, and see if the comment posted successfully before re-posting it.
I'm deleting most of the duplicates, so it's not a big deal. Just letting you know that in all the cases I've heard about where there's been an error, the comment did post successfully before the error happened.