In an interview with the author of How to Be a Working Comic: An Insider's Guide to a Career in Stand-Up Comedy, Tommy Smothers reveals what led up to the infamous "Troubadour" incident.

After the cancellation of the Smothers Brothers TV show, Tommy performed as a solo act:

I had been hanging around with Harry Nilsson and and working on stand-up material by myself. [...] It was in the early '70s and I was in New York doing a show at the Cellar Door[1]. I got there and they told me some guy named Harry Nilsson says he's here and wants a ticket. I had just left him in Los Angeles, but he had flown out on his own.
Well, I bombed something terrible. [...] My timing was off. I had to do two shows a night for a week and the first two nights Harry was up in the balcony. I asked some questions and the audience would respond. Pretty soon he started heckling, and the shows got pretty funny.
About two months later, we were playing the Troubadour in Los Angeles and Harry called John Lennon and said "Tom loves to be heckled. You gotta save him." So they came in all juiced-up and they got pretty vulgar. So it turned into a kind of mini-riot. We all got over it, but it was pretty exciting.