I have often found that I now understand something I read or saw years ago and did not understand then. For example:

• In Childhood's End by Arthur C. Clarke, Karellen's initial speech was in “English so perfect that the controversy it began was to rage across the Atlantic for a generation.” More recently, I realized the controversy was whether he was using a British or American accent.
• Prelude to Space, also by Clarke, mentioned “Britain's last millionaire.” At the time, it was expected that confiscatory income taxes would be the wave of the future and eliminate extremes of wealth. (My earlier post on confiscatory income taxes and SF is here.) By the time I read it, it seemed a comment on de-industrialization instead.
• Yet another Clarke story, “Silence Please” mentions the well-known composer Edward England. At the time, I hadn't heard of Benjamin Britten.
• The People of the Wind by Poul Anderson mentioned that there was a human precedent for the political structure of the Ythrian Domain but it was a bloody failure. Much later I found that it resembled the constitution of the late, unlamented Soviet Union. (A similar system was used on the planet Alphanor in The Star King by Jack Vance.)
• In the story “5,271,009” by Alfred Bester, the main character was temporarily restored to sanity by what was called “niacin plus carbon dioxide.” I later realized that it was a justification of smoking tobacco. Niacin is also known as nicotinic acid.
• In “Will You Wait” by Alfred Bester, the Devil was running for Congress. Alfred Bester's Congressman at the time was John Lindsay.
• In “The Foundling Stars” by Hal Clement, “‘Nineteen decimals’ had been a proverbial standard of accuracy for well over a century;” and that's because it's 64 bits (currently known as double precision).
• The ending of “The Cabin Boy” by Damon Knight turned out to be an allusion to a sea chantey I will not repeat here.
• In Country Lawyer by Bellamy Partridge, the career of a country lawyer included the last will and testament of a lady of ill repute. At the time I read it, I had no idea of what was meant by “Everybody in town knew what she was, thought of course some of the men knew better than others.”
• In Mathematics and the Imagination by Edward Kasner and James R. Newman, the examples of the common use of the word “probably” included the following:

  Money quickly, abundantly, and mysteriously earned during prohibition (it was judged, without consulting Bradstreet's) was probably the fruit of bootlegging.

  This was probably an allusion to Joe Kennedy.
• When I read Grimm's Story by Vernor Vinge, I didn't recognize the pun in the sentence “Just ventilating the structure required the services of twenty draft animals.”
• The episodes of Peabody's Improbable History ended in puns that I was too young to understand the first time I watched it. For example, in one episode, King Charles get his head stuck in a beehive because everyone knows he was bee-headed.
• The first time I read 1984, I didn't realize that the name “O'Brien” meant he was a member of a formerly oppressed minority.
• 1984 also included allusions to stuff I hadn't heard about. It wasn't until I read Darkness at Noon that I understood some of the allusions.
• Similarly, Fourth Mansions by R. A. Lafferty included allusions to Teilhard de Chardin I didn't understand at the time.
• When I saw Ghostbusters, I didn't realize it was based on H. P. Lovecraft's work.
• When I saw the “Lemming of the BDS” episode of Monty Python, I hadn't heard of Marathon Man.