Language Log has a great entry analyzing the use of “happenstantially” and whether we can consider it a word or not.
Happenstance is a word because it’s in the dictionary, but happenstantially isn’t in the dictionary. It does show up in Google searches 300 times, but the New York Times has never used it.
Does it need to be in the dictionary for it to be a word? In Google? In the New York Times? Used by your friends? What do you think?