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.