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Here are a few common mistakes (based on British and not American English):
Right[Rong]
advice (noun)[advise]
advise (verb) [advice]
practice (noun) [practise]
practise (verb) [practice]
definitely [definately]
you're [your]
receive [recieve]
receipt [reciept]
compliment [complement]
complement [compliment]
principal [principle]
principle [principal]
lose (verb) [loose (adj or adv)]
maffs [maths]
sux [sucks]
great [gr8]
never the less [nevertheless]
(posted: 26 Apr 2011)
allot [a lot]
( bos 26 Apr 2011)
discreet [discrete]
(The Australian: 28 Apr 2011)
Also
I do not like
your coming here [I do not like
you coming here]
{not a spelling error}
Owing to the rain, I was late. [
Due to the rain, I was late.]
could
've [could
of]
whom should I speak to [
who should I speak to]
(bos: 17 Apr 2011)