unknown toolbar item “undefined”

• Open the file fckeditor/fckconfig.js
• Locate the line
  ['Style','FontFormat','FontName','FontSize'],
['TextColor','BGColor'],
['FitWindow','ShowBlocks','-','About',]
• Remove the last comma(,) from the line. The new line will now become ['Style','FontFormat','FontName','FontSize'],
['TextColor','BGColor'],
['FitWindow','ShowBlocks','-','About']
• Refresh the page (Ctrl+F5) and your problem will have solved.

Courtesy : Unknown toolbar item “undefined” : FCKeditor forum