I find that I can cope with student inattention, misunderstanding, or laziness effectively. What I find most difficult is student unkindness.
Some students come to college with an immature view of professors. These students are unkind, rude, undermining, and distracting. Their snide jokes tend to draw others into their disruptions. Whatever the professor is trying to accomplish in class, these students will take every opportunity to treat the professor as an inept outsider.
Because I extend myself to set up an atmosphere of courtesy and friendliness in the classroom, my response to rudeness has not been to call it out immediately, but to squelch its effects by passing it over as unimportant. If I call the rudeness out directly, students like the ones I describe will take it as an opportunity for argument. Threatening to drop their grade is not effective because they are not motivated by grades. Instead, they are motivated by obtaining the attention of their peers, and also by their professor's discomfort. It has been more effective for me to diminish student rudeness by ignoring it; normally it disappears -- I think because, if the student does something well, I immediately reward the good behavior with a compliment and the good behavior becomes more manifest because it is reinforced while the bad behavior is ignored.