They aren't invisible ..at least they weren't on my end when it happened but if you have a lot of text and you aren't looking for them you will never notice them. I usually got them when I forgot to capitalize a proper noun and grammar check automatically corrected it.barnum42 said:The symbols that create the tongue smiley are ":" and "d" next to each other. Word must stick an invisible version of this in the text.
Personally I use the spell check plug in for Firefox. Saves the trouble of messing about with Word.