What's a joke or reference in a piece of media that you saw but didn't understand until years or even decades later?

In A Bug's Life, the circus ringmaster is a flea. It's a flea circus.

Admittedly I had heard the term "flea circus" my entire life but only this morning looked up what it actually was. I mean it's pretty straightforward, a bunch of fleas made to look like they're in a circus, but I was missing the history and context.