Go to this answer on a question
On that post click on "add/show X more comments" then immediately click "add/show X more comments" on the post directly above it.
A box pops up "You may only fetch comments every 2 seconds (click box to dismiss)"
When you click the box to dismiss, you discover that the "Add comment" box has appeared, but the additional comments that you tried to view are nowhere to be found.
You have to reload the page to find them.
I've reproduced it on both FF 126.96.36.199 and IE 7 on my Windows Vista box