Aluminum Christmas Trees: Modern readers might be confused when Cassandra mentions "mannequins" walking around, but at the time, many high-class shops employed live models to stroll silently about while wearing the latest fashions.note Ordinary, static mannequins did exist; the living ones were only to class the place up.
Hilarious in Hindsight: Cassandra and Thomas lock their father up and refuse to let him out until he has started a new book. This is treated as pure slapstick. Several decades later, Stephen King would use the same concept but played for horror instead.
Values Dissonance: While Mortmain's behaviour towards his children is mainly harmless, if pretty cold and distant, some of his actions (such as getting mad at Cassandra and pushing her into a wall) definitely don't go over as well today as they would have in the 1940s.
On a lighter note, the Mortmain girls treat Simon as an oddity because he is a young man with a beard, and he is described as becoming markedly more handsome when he shaves it off. This can come off as silly in an era where beards have become a lot more fashionable even among young men.
Perhaps less a Values Dissonance than a Do Not Try This at Home, Cassandra shares her chocolate bar with her dog. While a very, very little chocolate would not kill a dog as large as Heloise, giving one's dog any chocolate is discouraged now, partially because of the immediate health risks and partially because acclimating a dog to an occasional taste of chocolate might cause him to sniff out one's stash and overindulge to the point of serious harm or death. It's a notable lapse considering that author Dodie Smith was a lifelong dog lover with a great deal of practical knowledge about dog behavior and care, but at the time the book was written, the health impact of chocolate on dogs was not established and Smith would have had no reason to doubt the common opinion that it was okay to give your dog a chocolatey treat.
As a lady of society, one of the things Rose must learn is how to smoke. Social smoking was taken much more lightly than it is today.
