I think that has something to do with the spell checking part of FCKEditor.
Maybe give this a try...
http://stackoverflow.com/questions/2944509/how-to-disable-ckeditor-3-auto-spellchecker
Or maybe try switching to one of the other HTML editors.
Or to display just plain text for the extra fields values, in skin/common_files/modules/Extra_Fields/product.tpl
Replace...
{$v.field_value}
With
{$v.field_value|strip_tags:true}