I know my limits, but I also know that I have limits that I don't know. Complete self-knowledge would require both knowledge of my known limits and knowledge of my unknown limits. Complete self-knowledge, therefore, is impossible.
(Note how 'I' is used above. It is not being used as the first-person singular pronoun. It is being used as a universal quantifier. As above used, 'I' does not have an antecedent; it has substituends (linguistic items) and values (non-linguistic items). The above use of 'I' is a legitimate use, not a misuse.)