Similarly to "ensure"/"insure", "accept" and "except" it often seem confused; I've seen this even in professional writing.
and, in a similar vein/vain (wink):
"compliment"/"complement".
"lightning"/"lightening"
"cord"/"chord"
And is it "dependent" or "dependant"?
The old "fewer"/"less" chestnut.
"discreet"/"discrete"
"phase"/"faze"
The old "can"/"may" chestnut, as in, "Can I help you?"
"founder"/"flounder"
"climactic"/"climatic"
"council"/"counsel"/"councillor"/"councilor"/"counselor"/"counsellor"
"grisly"/"grizzly"
"homogeneous"/"homogenous"
"laid"/"layed"/"lain" - this is fairly complex. "He had laid on his bed all day," is actually wrong, as is, "He had layed on his bed all day." The correct one is, "He had lain on his bed all day." This is wrong all over the place.
"loath"/"loathe"
"miniscule" is wrong; it's "minuscule".
pronunciation of "mischievous" as if it's got four syllables.
"peninsular" is wrong; it's "peninsula" (for the noun; the former is the adjective form).
There's a difference between "phosphorus" and "phosphorous". Apparently 90% of the uses of the latter are wrong.
"practice"/"practise"
"wrack"/"rack"
It's "restaurateur" not "restauranteur"
The old "shall"/"will" thing.
"stationary"/"stationery"
"calender"/"calendar"
The old "who"/"whom" chestnut.
"should"/"would", as in, "I should/would like to do that."
The old, "If I was a rich man," thing. It should be, "If I were a rich man." The old subjunctive.
I would hazard/hazzard a guess, there are very few people in Britain who know how to always use all the above forms/alternatives correctly (I'm certainly not suggesting I do!) You'd have to be professor of English at Oxford or something.