In Oh Brother, Where Bart Thou? (2009), Marge and Homer both agreed that they didn't want another kid.
In season 11, after Apu and Manjula have octuplets, both Homer and Marge acknowledge that he is sterile from working at the power plant.
The broadcast syndication number is PABF18, which also happens to be the license plate number of the new car Marge buys for the family.
The title is a reference to "Adventures in Babysitting".
Milhouse describes the treehouse as a "treehouse of horrors," referring to the annual Halloween episode.