Forgetting to enter an exit date for withdrawn students, or entering an exit date for students ending the year normally (which schools/conferences should NOT do)
Not keeping the section number of a class consistent from term to term. Please keep the section number of a year long class the same from one term to the next within a single school year.
Not entering staff data, especially FTE and title.
Entering the wrong flavor of grades for classes vs. skills, appropriate by grade level. Be sure to check with the NAD report card site to make sure you are using the correct grade and skill scales.