Although nobody can say for certain, one of the best answers appears on a Word Trivia site, quoted below:

The words most likely to be misspelled (ratio of incorrect to correct spellings) according to a study of Usenet traffic several years ago were DUMBBELL, OCCURRENCE, MEMENTO, FRUSTUM, COLLECTIBLE, AMATEUR, DAIQUIRI, PASTIME, ACCIDENTALLY, PLAYWRIGHT, EMBARRASS, ACQUIT, HARASS, and PRONUNCIATION.

The same study showed these words most frequently misspelled (by gross count): RECEIVE, A LOT, AMATEUR, SEPARATE, REALIZE, THEIR, DEFINITE, INDEPENDENT, WEIRD, EMBARRASS, ARGUMENT, NO ONE, ACQUIRE, ACCIDENTALLY, OCCURRENCE, COLLECTIBLE, RIDICULOUS, MANEUVER, LIAISON, GAUGE, ATHEIST, GRAMMAR, SUPERSEDE, KERNEL, and CONSENSUS.

The study does not reveal common errors such as IT'S/ITS and YOUR/YOU'RE.

Another study of misspelled words at http://www.barnsdle.demon.co.uk/spell/error.html shows the following words (or forms of these words) as the most frequently misspelled: MINUSCULE, MILLENNIUM, EMBARRASSMENT, OCCURRENCE, ACCOMMODATE, PERSEVERANCE, SUPERSEDE, NOTICEABLE, HARASS, INOCULATE, OCCURRED, SEPARATE, EMBARRASS, PRECEDING, INDISPENSABLE, DEFINITELY, PRIVILEGE, GAUGE, QUESTIONNAIRE, EXISTENCE, MINIATURE, PRECEDE, WEIRD, RHYTHM, SEPARATELY, CONSCIENTIOUS, MISSPELL, HIERARCHY, GRAMMAR, CALENDAR, and WITHHOLD. The author of the website also points out, "Anecdotal evidence and personal observation indicate that a few other cases where a 'non-standard' spelling is frequently used on the Internet are alright for all right, alot for a lot, and it's for the possessive its. However, search engines don't look for extremely common words (asking one to search for posts with the word the would turn up just about every post ever made) and looking for two-word phrases (all right, a lot) is trickier than for a single word."

Some of the words in the above paragraph have alternate spellings in some dictionaries (such as "miniscule" and "millenium").

Addendum:  Definitely and Defiantly

We've seen numerous instances lately when definitely has come out as defiantly:  "We will defiantly come to the wedding," "I will defiantly attend class next week."  Here's how those often humorous mistakes occur.  The writer most likely uses definately, a common misspelling of definitely.  Spell-check spots the error and offers a list of possible corrections, the first of which (in Microsoft Word) is defiantly.  The writer okays the first choice and moves on, unaware that, though the spelling is 100%, the sense has changed completely.  Why Word does not offer definitely as a first choice is beyond us, but this is certainly not the only example of a case in which one needs to look closely at spell-check's suggestions before blindly accepting the first one.