A couple of things, most notably, people over 12 who don't have the excuse of being dyslexic, who can't spell for toffee and have poor grammar, especially people who can't tell the difference between your and you're!
What also winds me up immensely, is when I'm watching WWE, and a wrestler punches his opponent with the left or right fist, the commentators say "oh what a right hand!" when the guy punching is CLEARLY punching with the LEFT hand! I mean yeah OK up to about 10 years ago I had difficulty telling my left and right, due to poor co-ordination, but now I know my left and right, this winds me up when they do it.