Moved from http://blogs.msdn.com/vijgang

You might face a peculiar problem when you implement the custom dictionary functionality in SharePoint. This functionality is documented under "Plan dictionary customizations" section of article http://technet.microsoft.com/en-us/library/cc263367.aspx

All the words in the file uploaded the first time are recognized by the Spell checker. But if you make any modifications to the ‘custom dictionary.txt’ file and upload it again, the spell checker might not pick it up.

To workaround this issue, you may clear the system temp location - %systemroot%\temp location and try the spell checker again. You may also search for this custom dictionary file saved in the temp location and delete that single file (name is in the format GUID.txt)