I cringe when I see then used as than, or the other way around. They're, their and there get me too. Lastly, "should of"...it's should HAVE. I should have ignored this post and not answered.
Spelling mistakes (typos) don't bother me as much as grammar mistakes do. I get a lot wrong but like to think I'm not totally out of the ballpark. I think it just boils down to laziness but sometimes I truly have no clue what people are even trying to say.