This error kept appearing for the longest time, and my initial efforts to fix it by googling for a solution had failed. I decided today I would fix the error, even though I don't know much about how gnome-panel works.

The error message said "Some panel items no longer available" and was followed by an empty list. I checked in ~/.gnome2/panel2.d/ and looked over the launchers folder. Interestingly, deleting all the items from that folder only made some of my panel icons (aka applets) disappear, but not all of them. Time to inspect the dread gnome registry.

Now, I understand the advantages of a registry system, but when the gnome project adopted the registry, I got shudders seeing this fine F/OSS project emulating one of the most painful, problem-causing features of the proprietary vendors' systems. And, lo, here is the problem.

You can edit the gnome registry through System Tools -> Configuration Editor. Spilling out apps -> panel -> applets" I saw applets numbered 1 through 6. Looking at their names, I saw one which I knew was no longer on my system "MailCheck". I renamed this one "OAFIID:GNOME_MixerApplet" and gave gnome-panel a sighup. The error was gone, and two mixer applets were now on my launcher. Deleted one, and now, finally, my launcher launches without launching an error. Happy me.