Dean is shown wearing his lilac nightgown (to match the '50s aesthetic Mrs. Butters brings), which he wore in the fan-favorite episode "Scoobynatural"
Sam is shown wielding Mjolnir, the hammer of Thor, on a hunt. Mjolnir previously appeared in What's Up, Tiger Mommy? (2012) where Sam used it to kill the Norse god Vili.
Included in the picture of the Men of Letters is Cuthbert Sinclair. Mrs. Butters also mentions Mr. Markham who appeared in The Werther Project (2015) and Larry Ganem who appeared in As Time Goes By (2013).
In this episode Dean yells "we got one" just like the Ghostbusters do when they get a call.
This episode marks the first appearance of a wood nymph, which is another term for a dryad in Greek mythology. Drys signifies "oak" in Greek, and dryads are specifically the nymphs of oak trees, but the term has come to be used for tree nymphs in general, or human-tree hybrids in fantasy.