People don't seem to realize that most of these errors are typos, not evidence of poor grammar. The words that are the biggest problem for me are "its" and "it's". I understand these words perfectly, and yet, every time I need to type one of them, there's a 50% probability that I'll end up typing "its", and a 50% probability that I'll end up typing "it's". My brain imagines the sound of the word, and then my fingers type one of the words that sound like that, before I have even realized what I'm doing.This one is so pervasive and it absolutely drives me insane: the use of your instead of you're when appropriate.
There is however no excuse for "should of" instead of "should've" or (better) "should have".