The fundamental parts of the educational processes are teachers and students. The study aims to determine the common mistakes done by teachers in the classrooms. The research was conducted to 278 third year students in the departments of English Language Teaching, Turkish Language Teaching, and Fine Arts Teaching in 2012-2013 academic year. The research was conducted through the descriptive model in the survey research types. "What are the most common mistakes done by teachers in classrooms?" was asked to teacher trainees in questionnaire. The analysis was done by the content analysis among the qualitative research techniques. Findings are classified in the themes of "classroom management and discipline, teaching, communication, personal qualities, ethics, measurement and assessment, and other mistakes". Hence, many problems such as "failure to establish the authority and compensating it through being harsh toward students, being much more authoritarian and disciplined, seeing himself/herself as the authority source" emerged in this perspective. Furthermore, many inadequacies such as not being positive and gentle, negative attitudes, choosing attitudes directing students in a difficult situations, anger, having peaceful mood, threatening students with bad grades, claiming that he/she is the best, reflecting psychological problems in the classroom are determined at the end of the study.