Just recently I've been noticing this.
On SO, sometimes when I click "add/show X more comments", the add comment edit box is shown immediately, but the additional comments aren't shown for several seconds. There's no indication that the browser is accessing the site during this time.
Maybe it's my browser/environment (IE 8 on Windows 7 Enterprise, on a LAN with a proxy on the other side of the country (USA)). I can't remember noticing it on Chrome but I haven't accessed SO from home much lately.
It doesn't happen most of the time and I can't reproduce it at will. It's just weird.