My most common mistake

For homework over the weekend we were assigned to read "Under the Grammar Hammer" by Douglas Cazort. He mentions all different kinds of common grammatical errors but focuses on the 25 most common. After class we were told to pick the error that we make the most in writing and I picked out two that are quite common in my writing. The first one that I am guilty of making is the "unnecessary shift in tense" which happens all to often. A technique that I learned in high school that helps me avoid this mistake is to read through an essay and underline all of the forms of the verb "to be". At the end of the essay I go back and look at all of the underlines verbs and make sure the tense is consistent. The seconds mistake that I tend to make is the "unnecessary shift in pronoun". This is by far my most common mistake because it is fairly easy to skip over in proofreading. It usually happens when halfway through my writing, I realize I have been saying you, or one, and I decide to change it and don't go back to change them all.