I've had two instances of this in the last few days.

Clicked "submit" in a quick reply and nothing happened (including no turning wheel in FF to show that there is data traffic).
Switched to advanced mode and clicked submit--and then wound up with a double post.

EDIT--More data:
After clicking submit (and nothing happens), I go to advanced and note that my submission is there. (Before switching to advanced, nothing happened for ~ 2 minutes after clicking submit.)

I've had similar response problems. I just upgraded to Firefox 2.0.0.6 so I was blaming that or my ISP. I've also been getting TCP/IP problems such as duplicate packets et. al. so that could be my ISP or my hardware. If other people are having response problem with LQ then I have one more source of network heartache and despair.

I've also seen a lot of incorrect marking for new posts in subscribed threads. Often a thread title will be in bold font when the last post was mine, and a thread title will not be in bold font when there is a new post that I have not read.

No show stopper problems though. Just quirks.

The issue with posting in quick reply has been solved on many machines by clearing the browser cache... probably some of the javascript elements are being cached by your browser.

This does seem to be a caching issue. We'll push out a no cache directive for a while to help alleviate the issue, but a shift-refresh has solved it for everyone so far. If this doesn't work for you, please let me know.

--jeremy