"u", "ur", etc. in posts and even post titles. Nominative case personal pronouns as objects of predicates or prepositions. Inability to conjugate past and present perfect tenses. Writing "should of" for "should've". The list goes on, even though this stuff is taught over and over, for so many years in school. Most often I hear that from the sportscasters on TV, who seem to consistently have these problems, and sometimes I just get fed up. Oh, yeah, TV folks also seem to think they can use "with" as a predicate.