Light Plane: Transition to Right-Seat Flying: How to Prepare, What are Common Errors?
First, congratulations!  Earning a CFI certificate is a real undertaking.

For a literal textbook answer to your question I suggest opening this PTS pdf (assuming FAA CFI) and searching for "common errors" until you find yourself in relevant (to you) sections.  There are many examples.

On a personal note, I found that talking through every action made maneuvers go smoothly and let the instructor/examiner know that what I was doing is intentional.  It also forced me to follow my own directions so, since I knew what was supposed to happen, I just said it aloud and then did it.  It sounds silly but it worked well.

The transition to right seat flying is simple.  You'll have it down in an hour or two.  Nothing to it.  It's actually kind of anticlimactic.  Which plane are you using?