I sometimes have issues with "recommend", where I'll instinctively type "reccomend", even when I know it's not spelled that way.
"unnecessary" is another one where I sometimes spell it "uneccessary" or "unneccesary".
I also find the need to recite the "I before E except after C" rule more often than I'd like to admit.
Heh, I used to spell "originally" as "origionally" for the longest time, until around 2005, when I arbitrarily decided to use spell-check. Boy, was I embarrassed. The sad thing is no one ever corrected me. I wish they had.