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This is the first time we see Dean express vulnerability and strong emotion: at the prospect of returning to Lawrence, being in his old house, feeling lost as to what to do, trying to reach his father, and finally confronting his mother's spirit.
The number for Dean on John's answering message was 866-907-3235, which for a time was an operational number with voice recordings by Jensen Ackles. It is no longer in service.
Sam tells Sairie the exact words that his dad told Dean when their mother died.
This is the first episode to explicitly state that Sam has some kind of psychic powers.
The Cymbal-banging monkey is a reference to Stephen King's Short story "The Monkey", first published in 1980 "Gallery Magazine," and later included in the book 'Skeleton Crew" (1985).
There are no deaths and only one major injury in this episode. It makes it the first episode in the series with no fatalities. (Although the plumber is shown losing his arm in the garbage disposal, there is no police presence, none of the characters react to it as though he'd died, and he's well enough to try to file a lawsuit against Jenny, as we hear on the phone.)