Old Shame: On a late-'80s radio interview, John Cleese spent several minutes thoroughly denouncing the show and said he thought it was a waste of his time to be doing it. In his 2015 autobiography So, Anyway, he said that his mixed view of the series was the audience's fault for laughing and cheering so loudly whenever a recurring character or catchphrase was recited (a practice not unique to ISIRTA and one that Cleese deplored even then), leading to the overuse of such devices at the expense of fresh ideas.
The Pete Best: In the first series, the primary cast comprised Tim Brooke-Taylor, John Cleese, David Hatch, Jo Kendall, Bill Oddie, and... Anthony Buffery, who had written some of the sketches for A Clump of Plinths/Cambridge Circus but chose to focus on his academic career when the revue went to London, whereupon he was replaced by Graham Chapman. He left the radio version after one series and was replaced by Graeme Garden (who was never part of the Cambridge Circus cast as he was a year behind the others at Cambridge, but did write some of the sketches). Buffery went on to appear with Graeme, Bill, Terry Jones, Michael Palin, and fellow Cambridge Circus veteran Jonathan Lynn in the TV sketch series Twice a Fortnight before giving up entertainment to pursue a career in psychology.
Talking to Himself: Occasionally, and usually Lampshaded when used. A common situation in the "Curse of the Flying Wombat" serial from Series 4 was a conversation between recurring characters Tim Brown-Windsor and Lady Constance De Coverlet, both played by Tim Brooke-Taylor. One episode took it Up to Eleven by adding in Lady Constance's identical twin sister. Inevitably, Tim would slip up and use the wrong character's voice for a particular line.
Of the nine series, only five featured the full core sextet of cast members. Graeme Garden was not part of the Series 1 cast, and also sat out Series 4 and 5 (except for "The Inimitable Grimbling" when he replaced an ill John Cleese) due to the demands of the clinical phase of his medical studies, while John Cleese was absent for Series 2. The only performers present for every episode were Tim Brooke-Taylor and David Hatch (Jo Kendall and Bill Oddie were always part of the regular cast, but each missed at least one episode due to illness or other commitments).