On an up-to-date Ubuntu Dapper i386 install adding xpenguins-applet to the panel freezes the Add to Panel window and freezes all panels. Even after a reboot the panels stay frozen and the only way to cure it is to remove xpenguins-applet.
Steps to reproduce:
1) apt-get install xpenguins-applet
2) Right click on your panel and select Add to Panel...
3) Select the XPenguins applet and click on Add
Expected results:
1) An applet attaches to my panel allowing me to control some zany penguins doing various entertaining activities on my desktop.
Actual results:
1) The Add to Panel window freezes up
2) My panels freeze up making them useless
3) Killing xpenguins-applet doesn't help (top tells me that it respawns)
4) Rebooting doesn't help since gnome-panel tries to add the applet to itself again
I unfroze my panels by apt-get remove xpenguins-applet so that it couldn't respawn and then killall xpenguins-applet. Removing the applet manually from the gnome-panel applets configuration might have worked too but I don't know where that is.
Other info: This may be related to bug #6615 but I'm not certain since it's not very well written.
In the spirit of completeness I feel I should add that I have a few customisations to my panels so I'm not using a fresh desktop set up. The panel I tried to add the applet to is on the right hand side of my desktop and it has been given some transparency.
I think that covers all the bases.