Words that always make me stop and think when I try to write or type them are
Doctor and Computer
I never know for certain which of the two has an 'er' and which has an 'or' at the end. Trying to say them out loud doesn't work. Nor does writing both spellings and picking which one is right. If I think it matters, for example when putting them on my blog, I might even have to type them in WORD and use the spell-checker just to check.
I also can't spell 'assembly'. As i did in the post I wrote a few weeks ago, I always want to put an 'e' in between the 'b' and the 'l' (assembely). That's why I made myself leave the mistake in that post - to make me cringe everytime I see it, in an attempt to learn how it ought to be spelt.
I can now spell 'awkward' though, once I stopped trying to put a 'q' in it.