People who don't know the difference between 'of' and 'have'. You wouldn't write: 'I want a cup have tea', so don't write: 'He should of got it years ago'.
Indeed. Add to that people who do not know the difference between 'fewer' and 'less'. I may have less food on my plate than you, but you have fewer chips. And people who do not realise that the letter 'H' is not aspirated when used in acronyms.
I could go on. I often do...