We all make mistakes, God knows. I misspelled my graduatethesis adviser's name on every sheet of my thesis proposal. Nevertheless, such mistakes are to be avoided if possible. Here are some of the most common ones I encounter in papers from freshman comp to graduate seminars.
This one is pretty simple once someone points it out to you. Here is the exception to the rule in English about how we make possessives and contractions. We usually make the possessive by adding apostrophe (') s. We make a contraction in the same way. Hence, you would think "it is" and "of it" should both be "it's." But this way confusion lies. So once upon a time someone arbitrarily decided that "it is" has priority and deserves the honor of the apostrophe. "Of it" must therefore be satisfied to live without one. So think of meaning and remember:
it is = it's of it = its
As for "its'," a construction I have indeed seen, try to imagine the sentence that would need a possessive of the plural "its." For the only one I can imagine, the word "its" would have to appear several times on the blackboard in green chalk, and someone