It happens when I have written a post or a comment, and then submit it. If the server load is high, it does not get submitted right away and load the result, so I am left wondering if it worked at all.
Press submit again, ah yes now it worked. But in fact the first submission also worked, I just didn’t see it, so now there are two, and I don’t even know!
With the amazing performance improvement launched today, I would guess that this sort of problem will ich be rare from now on.