The symptoms you describe may indicate that the comment has been spammed. This can be for a number of reasons, determined by each blogger's individual anti-spam plugins activated, discussion settings, and comment content. 

If a comment does not appear, and no message is displayed, it is likely in the bloggers spam folder. If the same comment is posted again, the reader will receive the duplicate comment message.

Depending on a blogger's discussion settings, including links in a comment can often cause it to be spammed. The default setting for all blogs is for the first comment by any reader to require moderation, with future comments from that reader posting automatically. So no, readers will not always always get a confirmation that a comment is awaiting moderation. And a comment is considered duplicate if the content is identical, and comes from the same IP address whether logged in as a different user or not.

Feel free to provide a link to specific post(s) if you would like us to confirm any of the above. Anyone who experiences the same and wants to ensure their comment is read should send the blogger a PM suggesting he/she approve their pending comments or check their Spam folder from the Comments tab in their dashboard.

Hope this helps!

riosmom said:

I would try and post a comment only to have a "sorry, that page could not be found" page show up…

If this message appears, reloading the page will usually result in discovering that the post did indeed publish successfully. It's usually just a matter of intermittent network latency.

To troubleshoot the other issues, we would need more details about the exact time, any error messages, browser being used, and IP address of the user. Anyone experiencing such difficulties should please start a new topic since this thread about blog comments has been resolved.

Sorry for any troubles, we have an update pending which should vastly improve overall performance around here. Just waiting for WordPress 3.3 to drop.