In "Smurfing for Ghosts", Brainy's line, "Go ahead, smurf my day," is based on Clint Eastwood's famous line from Dirty Harry (1971).
"Smurfing for Ghosts" uses the visual gag that TV Tropes calls "Scooby Dooby doors" when Peewit, Brainy, and Clumsy get chased by ghosts inside Quarrel Castle.
The episode "The Gargoyle of Quarrel Castle" marks the first appearance of Selwyn, Tallulah, and Tharp.
"Smurfing for Ghosts" is the second time Peewit appears without Johan, meaning this isn't considered a Johan And Peewit episode at all.
The episode "Smurfing for Ghosts" is an obvious homage to Ghostbusters (1984), featuring a "Jimmy Hart version" of its theme song in the episode's soundtrack.