First off I have only seen this happen on large threads in the 60k+ replies range. In these threads it seems to pretty regularly occur that when the quick reply form at the bottom of the page is used to submit a reply that the gray progress indicator pops up in the upper right as would be expected for a moment, then it disappears as if the action is done but the page does not update with the new post info as would be expected. BUT, the post does actually get posted if the page is manually refreshed for new information.
We have only seen this behavior in these very large threads though, so it seems that this has something to do with thread size. Let me know if you would like some links to example threads of this bug.
Can you reproduce it yourself? If so, is there anyway we can see what is being returned by the server via the Ajax request? You can do this through the dev tools > network panel in something like Chrome or via Firebug's console.
Internal server error, in testing this out I also went back to delete some posts as well (hard delete) and got an internal server error upon submitting those as well, though the delete actually did happen similar to what is happening with the posting there.
And it appears that the following logged php error is related:
[09-Aug-2011 16:10:15] PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 72 bytes) in .../forums/library/Zend/Db/Statement/Mysqli.php on line 304
A bit more info that might be useful as, as mentioned before the posts are making it into the thread though are resulting in a internal server error. Users are reporting that they are not getting notices that are normally triggered upon posting, specifically notices for being quoted or watched thread notices on the effected thread.