Actor Allusion: In "The Hold Up," Mr Harman comes up with a plan with Mrs Slocombe and Mr Humphries to rescue the others by dressing up as the dreaded Gumby Gang. As Pa Gumby, Mr Harman is dressed up as a character he was most famous for doing years prior to the show: a shady "spiv" character, complete with pencil moustache and wide tie.
Dawson Casting: To a minor extent. Mr. Lucas was supposed to be in his mid-twenties when the show began, but the actor, Trevor Bannister, was 36 years old. In fact, he was only a few months younger than Nicholas Smith, who played Mr. Rumbold, and nearly a year older than John Inman, who played Mr. Humphries. Inverted for "Old" Mr. Grace who was played by Kenneth Waller age 54; his younger brother was played by Harold Bennett, who was 28 years older.
Joanna Lumley appears as a salesgirl in "His and Hers", and a German woman in "German Week".
When the series ended, Wendy Richard (Miss Brahms) began her 22 year stint as Pauline Fowler in EastEnders.
Carol Cleveland has a cameo as the mother of an obnoxious little boy who won't put his squirt gun away.
The Other Darrin: Mrs. Peacock, who makes appearances in a few episodes, was played by two actresses.
Old Shame: The 1981 Christmas Special "Roots?" (named after the seminal US mini series Roots) has the main cast performing a routine in blackface obviously inspired by the old Minstrel Shows type routines - this episode is rarely repeated as a result. You can see the offending routine at 8:30 in the link 
Playing Gertrude: Nicholas Smith (Mr. Rumpbold) is only nine years older than Wendy Richard (Ms. Brahms.)
Reality Subtext: The series' creators have implied that Mr. Humphries was promoted to senior salesman because so many of the elderly actors who had previously appeared on the show died (Arthur Brough, Harold Bennett), retired (James Hayter), or just didn't work out (numerous others). The younger, healthier Inman was already known to the audience and reliably funny, so the producers felt they could count on him to stay around for several years. Thus, they made Humphries the senior character, and never cast a new middle-tier salesman.