In the 2009-10 TV season, NBC faced a late-night ratings disaster by giving then-Tonight Show host Jay Leno his own talk show, titled The Jay Leno Show, while Conan O'Brien was off filling in for him, hosting The Tonight Show with Conan O'Brien, which aired from 6/1/09-1/22/10 before Leno returned. Given that this was the shortest incarnation of The Tonight Show, any of its 145 episodes are tough finds, especially an episode that was never released in any form.
On September 25, 2009, he was performing a skit with Teri Hatcher of Lois and Clark: The New Adventures of Superman and Desparate Housewives fame when he slipped and hit his head on the floor, resulting in a mild concussion followed by an examination at a local hospital, where he was later released. Due to the incident, the episode set to air that night was left unfinished, with a rerun airing in its place. O'Brien joked about the incident and played a clip from the abandoned show the following night, this clip being the only footage of the episode that has surfaced to this day.
Conan O'Brien's Concussion in HD 9 28 09
The only available footage of the episode.