From:
  http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=116741

O.K., it took me a while to get the exact sequence of events, but
here's how I can do reproduce everytime on 2 different up-to-date FC1
boxes.

- Create a new panel.

- Right click panel and adjust these properties:
   Uncheck "Expand"
   Check "Show hide buttons"
   Check "Arrows on hide buttons"
   Change Orientation to Right

- Middle click on top hide button and drag the panel to the uppermost,
  rightmost corner of you desktop.

- Click the bottom hide botton. (panel should go to bottom of screen,
  with only the top button showing, covering up the bottom panel)

- Middle click and drag the panel left so that the panel changes to
  just a top hide button and then drag it right back to the very bottom
  corner.

- At this point, you either have a panel without a bottom button, or
  you just have a top button.  Quickly click the top button and voila!

FYI, I've gotten something similar in the top right corner as well, by
holding down the left mouse button on the bottom hide button when the
bottom hide button is the only thing showing.