Shining, When I think of mistakes, there are "positive and negative" versions of this definition. In the positive sense, trying new things or finding too little (or too much) time allocated for a segment of instruction could be cited as a mistake that a teacher learns from. On the negative side, making an intentional poor judgement or demonstrating unprofessional behavior would be examples of preventable mistakes, and should be avoided. I think a good rule of thumb is sticking close to the course objectives and planning the daily lessons (along with needed resources) carefully in order to assure the students gets all they need and the teacher achieve the desired learning outcomes. Barry Westling