I can understand people making mistakes because it's a forum, we post quickly, it's not formal English, etc.
Except things like 'would of' or 'should of', or 'your' instead of 'you're' aren't mistakes because we're posting quickly.
They are mistakes which indicate that people simply don't understand what they should be writing.
They are incorrect, and yes - they irritate the hell out of me. I don't give a shit what contractions you might use, what slang you might use, but these things indicate that you don't understand basic grammar, and I will judge you for that.