I have just received an e-mail which referred several times to a general practitioner's 'practise', and in one instance, a scheme the author opined would not work 'in practise' and in another, of a managing director's 'principals'.
Personally, I'm always more annoyed by those who mix up advice/advise.
But top of list are people who claim "I am a self-proclaimed [something]". I know that - you just proclaimed it. In fact someone on this very forum the other day wrote "I am a self-proclaimed pedant". I wanted to reply "As a self-proclaimed pedant, don't you see what's wrong with what you just wrote?" but the freshness was gone.