Complex sentence: It was his 12th appearance on a Letterman late-night show, but his entire performance was removed from the broadcast; until that point the only occasion where a comedian's entire routine was cut after taping.
Split into simpler sentences: It was his 12th appearance on a Letterman late-night show, but his entire performance was removed from the broadcast. Until that point the only occasion where a comedian's entire routine was cut was after taping.