Question of the Week: What's the Most Egregious Teaching Mistake?

I assert that there are 10 Major Mistakes That Most Teachers Make. If you could wave a magic wand, what one teaching mistake would you want to erase from the classroom?

If I could make one change it would be to erase campus politics and inter-teacher bickering. I feel that teachers have limited energy and unlimited To Do Lists, so there just isn't any room for unnecessary squabbling and backstabbing.