It occurred twice to me last month. The last time I recall that it worked was more than two months ago.
I however do see that the page body shifts a bit to the bottom to create room for the bar, but the bar itself doesn't show up at all. Too bad that I didn't check for any JS/CSS errors. I also don't have the playground environment to try to reproduce this (I don't think that we're allowed to abuse Meta for this).
Using Firefox 3.6.3 on Windows XP.
Update: this has occurred once again recently. This time the question got migrated while I was busy improving the question. I took a screenshot (note the blank space in top), but there were no JS/CSS errors.
The "new answer has been posted" notification has by the way always worked flawlessly. I just tested it directly thereafter without changing/restarting the webbrowser and here's a sceenshot as evidence.
Update 2: just wanted to inform that it works again! Last time I recall it didn't show anything in the room for the bar was two days ago. Was something changed with regard to this in the meanwhile?