My number one Pet Peeve: the incorrect use of the words Your & You're.
Folks I know it doesn't take a rocket scientist to get this right. I know this for sure! Don't even get me started on UR. Moving along. It's not that I don't make mistakes, I do, quite often as a mater of fact but this one here is just a no brainer. Your is exemplifying possession, ie your car, your idea, your mom, your notes & You're is a contraction of You are, ie you're amazing, you're missing a great party, you're going where?
If you pay just a small amount of attention your writing will improve greatly. Just a little bit of mindfullness goes a very long way.