What are the ten or 20 words in the English language that are spelled
incorrectly most often?

Hello.

There are two ways of looking at this question.

One way is to consider the words that are most commonly misspelled in
terms of the ratio for incorrect to correct spellings.

A survey of Usenet traffic several years ago revealed that these were
the words most likely to be misspelled:

Dumbbell
Occurrence
Memento
Frustum
Collectible
Amateur
Daiquiri
Pastime
Accidentally
Playwright
Embarrass
Acquit
Harass
Pronunciation.


Another way of looking at the question is to considered the most
frequently misspelled words in terms of gross word count (i.e., some
of these have a better chance of being spelled correctly on a
percentage basis, but are the most frequently misspelled because they
are used more often).

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
Consensus

Sources:
"Some Common Misspellings"
http://members.aol.com/gulfhigh2/words9.html
"Language Doctor"
http://www.britishcouncil.lk/english/Languagedoctor2.htm

-------------

Another study, using basically the same criteria as the first list
above, found that these were the 20 most commonly misspelled words:

"minuscule 68% of the time spelled otherwise
millennium 57
embarrassment 55 (embarrassing 35%)
occurrence 44 (occurring 37)
accommodate 40 (accommodation 39)
perseverance 36
supersede 35-50 (superseded 44)
noticeable 35
harass 34
inoculate 34

mischievous  32%
pastime 24%
occurred 31
separate 23 (inseparable 21%)
embarrass 30 (embarrassed 29)
preceding 22 (preceded 21)
indispensable 29
definitely 20
privilege 28
gauge 20 (gauges 25)
questionnaire 28 "

sources:  
"Common Misspellings and Dictionary Alternatives" by Cornell Kimball
[Journal of the Simplified Spelling Society, J26, 1999/2, pp3-10]
http://www.spellingsociety.org/journals/j26/misspellings.html
"A Study of Some of the Most Commonly Misspelled Words"
http://www.barnsdle.demon.co.uk/spell/error.html

-------
search strategy:
"common misspellings"
study, "the most commonly misspelled words"

I hope this helps.

What about "it's" instead of "its" or "your" instead of "you're".
These two turn up absolutely everywhere in the wrong context and it
drives me crazy!!

Stressedum,

That's a good point.  It's/Its & Your/You're are indeed frequently
misused. One problem with picking up those errors is that you'd have
to look at the context for each use since those are standard spellings
(just used in the wrong context).

Here's what one of the articles that I cited above has to say about the issue:

"Anecdotal evidence and personal observation indicate that a few other
cases where non-standard spellings are 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.
  In giving results for these searches, the search engine gives a
numerical count of all occurrences of a particular form - I simply
used those numbers, and did not usually need to read thru each
individual post. But to determine the rate for the possessive its, I
would need to look thru each individual post to determine whether it's
was used incorrectly for the possessive its or whether it's was used
correctly for the contractions it is/it has. So, I didn't search for
these three cases, but it may well be that all right, a lot, and the
possessive its are among the most frequently misspelled words in
Internet postings. (This situation - where the misspelling problem is
that its is misspelled as it's - is the opposite of the situation for
beginning spellers, whose frequent error insted is to misspell it's as
its)."
http://www.spellingsociety.org/journals/j26/misspellings.html

Successhotline-ga: Another common misspelling can be found in your question.

I often type 'thnaks' instead of 'thanks' ...

Can I join the list, please?

Many thnaks.

Hi, Monroe22

'Mispell' (sic) is an alternative to 'mispend'. See

http://www.onelook.com/?w=mispell&ls=a

So, how is a Researcher to know whether a word has been misspelled or mispelled?

Thnaks

Hi probonopublico-ga: My Webster (admittedly not the only
authoritative source)does not list *mispell* It seems intuitively
correct that the prefix *mis* should remain intact...but then, my
intuition often fails me. Thanks for your comment, and your scores of
interesting remarks in various areas.
monroe22-ga

100 Most Often Misspelled Words In English and more. 

http://yourdictionary.com/library/misspelled.html