The "Piano Lesson" segment was based on a real-life conflict between Louis C.K. and Marc Maron. C.K. and Maron had been roommates early in their careers and best friends. But jealousies ruined their friendship and they were estranged for many years. In 2010, C.K. appeared as a guest on Maron's "WTF" podcast in which they apologized to each other and became friends again.
During the conversation between Louis and Marc Maron, you can see an old picture of Louis and Marc together, hanging on the bulletin board on the wall behind Louis.