The British thing is one of my distinct problems. I learned to read living in the UK, so they tend to be my default spelling. And then sometimes I forget which is which. Some I just forget there is a difference.
And it's not helped (for me) by the fact that most stuff is set to American by default, so I find myself getting confused, and I've never lived in the US.
As to your original question, I misspell things all the time - my stupid fingers keep wanting to type random stuff. It's usually the small words - it, is, of, if, and so on - but I've recently noticed I keep spelling embarrass wrong, and any of its iterations, which is, well, a bit awkward. Thank goodness for spell checkers.