I have trouble with double letter words like necessary and professor. I always try to add letters like, 'neccessary.' Oh and this isn't a misspelling, but I also overuse commas by putting them in places where I would pause to speak. I'm sure it's annoying.
I get on to people for basic grammar sometimes, too (also annoying). For example, if someone doesn't know the difference between 'your' and 'you're' I attempt to correct them in the friendliest manner possible.