A picture of Eileen is revealed to be inside of the house after Rigby turned into it on the third short. Implying he have feelings for her but don't like to show it as of this point in the show.
"In the House" is a reference to The Evil Dead franchise.
It is revealed that the park has a giant hole where Muscle Man and Hi Five Ghost wreck cars in, which they call the Crash Pit. However, it wasn't proven to exist until "The Best Burger in the World."
The franchise this episode creates is a parody of The Simpsons' "Treehouse Of Horror" Halloween series.
When Muscle Man was skinned alive by the Halloween Wizard, his body accurately resembles the Colossus Titan from the anime Attack on Titan.