British researchers have concluded that the most commonly misspelled word in published documents and on the Internet is "supersede." (We're thinking on the Internet at least, they might have overlooked "ponography.")
In any case, here's a really interesting article that blames common misspellings on a variety of causes: misinterpretations of etymology and meaning, and errors related to pronunciation.
And here are the most commonly misspelled words, at least in published documents:
Five most difficult words based on wrong assumptions relating to content: supersede (precede), inoculate (innocuous), sacrilegious (religious), consensus (census), liquefy (liquid)
Five most difficult words based on a foreign language root: broccoli (Italian), haemorrhage (ancient Greek), connoisseur, manoeuvre, lieutenant (French) [NOTE: SOME ARE BRITISH SPELLINGS--In the U.S., you'd see hemorrhage and maneuver.]
Five most difficult words based on difference in pronunciation: conscience, indict, foreign, mortgage, phlegm
Phlegm! That's a word everyone should know how to spell. All you teachers out there, please put it on your lists.