Even the best teachers are not perfect. Thinking back to my own experience with my previous teachers, I realize that some of them had a bad day now and then. Or maybe I was the one having a bad day. In either case, it seems that most classrooms have some distractions* from time to time. Once I got "off track" during a lesson, it was really hard to catch up.
*Examples of distractions include situations where other students are moving or making noises, my own worries about friends or school, family concerns, being hungry, needing to go to the restroom, looking for a pen or pencil, or needing...