- In The Sea of Monsters, Annabeth regrets forgetting her special coconut sunblock from Medea's department store.
- The bronze dragon also makes its first appearance in The Demigod Files.
- A later plot point: Bob/Iapetus's first appearance was also in The Demigod Files.
- Also, while shopping in Medea's shopping mall, a bronze breastplate that is corroded with acid is mentioned along with other merchandise belonging to deceased campers. It likely belonged to either Silena or Clarisse.
- The Armor coupled with Aphrodite's conversation with Piper doubles as Fridge Brilliance when you realize that Aphrodite was most likely referencing Silena when she held up the armor.
- Nereus, the old man of the sea, last seen in Titan's Curse, makes a blink-and-you'll-miss-it but funny cameo in Son of Neptune.
- Vitellius, in Son of Neptune, asks whether or not Percy cleans stables during their first conversation. Since Percy's memory has gone, he can't remember that he did in Battle of the Labyrinth.
- In The Mark of Athena, Percy decides against telling some water nymphs who have been trapped that the nature God, Pan is dead, something he personally witnessed in The Battle of the Labyrinth.
- Also in Mark of Athena, Percy compares the problem with the doors of death to having a "dam hole". When the characters ask what he means, he says it was an inside joke.
- While in Camp Jupiter, Percy is told that all Demigods have a talent for Latin, and meanings sometimes just pop into their head. Percy isn't Roman, so this isn't true. But when Reyna introduces her metal dogs, Aurum and Argentum, Percy instantly translates them into "Gold" and "Silver" respectively. Why? All the way back in the first book, in the original series, Percy took a Latin class.
- Leo meddles with Coach Hedge's megaphone in the very beginning of The Lost Hero, and it reappears in the last book, still malfunctioning.
Continuity Nod / The Heroes of Olympus