- Life Imitates Art: There is no real "Angel" statue,note but in 2009, the Met acquired a small mysterious marble cupid likewise reputed but unconfirmed to have been created by Michelangelo. Both the statue and the circumstances of its acquisition are very similar to the ones attributed to Angel in the story, and the book itself references a long-lost Michelangelo cupid. In an Author's Note to the 35th anniversary edition of the book, the author herself remarks on the coincidence and invokes this trope by name. In 2012, the so-called "Manhattan Cupid" was confirmed as a genuine Michelangelo, meaning that Mrs. Frankweiler's long game finally ended.
- Technology Marches On: While even a museum like the Met wouldn't bother to put a security system on every exhibit, the rise of affordable and effective CCTV cameras and motion-activated alarms ensure that Claudia and Jamie wouldn't last a single night there now... much to the disappointment of modern children who still view the book as a perfect fantasy of running away.
- Values Dissonance: In the rise of Stranger Danger, Amber Alerts, serial killers, 24-hour news cycles, public knowledge of human trafficking, and other modern horrors, a pair of missing kids would be a much more serious matter these days. Up until the 1980s, you had to wait 24 hours to file a missing persons report, even if the missing person was a minor; this law was changed specifically in the wake of several high-profile child abductions that ended in tragedy. Even an eccentric millionaire like Mrs. Frankweiler would more likely call the police immediately rather than invite the kids to lunch, if only to clear herself of any suspicion.
Trivia / From the Mixed-Up Files of Mrs. Basil E. Frankweiler