It is mean to call someone out publicly.
Greg is a teacher and sees a student copying off of another student's paper, preparing to turn it in.
Greg wants to tell the student that cheating is wrong.
Greg meets with the student after class and explains that he can't accept homework that was copied from another student.
Greg tells the student from the front of class that copying answers is not allowed and that his homework will get a zero.
The student is disappointed but accepts the punishment.
The student gets very angry, defensive, and embarrassed in front of his peers.