The Update button in the bottom left (available after "Editing" your post) has the frustratingly annoying habit of not actually updating my post, forcing me to repost my changes. Sometimes more than once.
Aren't you using FireFox?
Some have commented on Update not working for them too, but I do not recall their browsers.
I have not encountered similar with Chrome (though there ARE some other forum oddities... )
Maybe other FF users can comment and be of help.
It has been my experience that if I repeat the process two or three times, it will eventually stick (Firefox 3.6.13 on Win2K).
Mine as well.
It should happen the first time, for every person, using any browser. That's my point.