When you add a launcher to the panel, (Ex: go to applications menu, right click firefox and 'add as launcher') and switch between task managers, using the alternatives option, said launcher will disappear from the task manager it was added to. Reproducible: Always Steps to Reproduce: 1.Add an application launcher to the panel. 2.Switch to a different task manager. 3.Switch back to the task manager you added the previous launcher.
This is a general problem with the Alternatives mechanism that applet settings aren't migrated. There was a duplicate bug report about this but I can't find it right now.
*** This bug has been marked as a duplicate of bug 355588 ***