If you created your own online help and created your own table of content. In certain cases where you have bullet points in your document, this is considered as special characters. This may corrupt the metatag in the htm file.
See screenshot below the description metatag takes the first 300 characters to be indexed (to be displayed as result text when you search on it).
This particular example ended with .. You should delete that last part it should end without any special characters.
Last note. There is an msdn article for troubleshooting online help.