Beloved ends on a note as strange and poetic as its whole with musings of whether or not the titular Beloved was ever really there and the repeated refrain that speaks to the rythm of, "It was not a story to pass on." Alluding to the nature of the novel and in turn breaking, or at least nudging up against, the fourth wall that thinly separates our concetion of fiction from our bare boned reality.
Tvtropes.org (an absolutely AWESOME website for understanding how stories work that you should totally check out sometime, largely for its previously stated awesomeness) defines the 4th wall as- "the glass on the front of the TV tube, the invisible wall at the front of the stage play set. This separation between the characters and the audience" In breaking the fourth wall, Morrison has made it explicitly clear to her readers that they are reading a book, a practice which could in theory alienate her readers from the characters they've been spending time with, largely just makes one appreciate even more just how much she has done to ensure absolute emotional realism on the part of her characters.
Despite the presence of ghosts and other near supernatural phenomena, Morrison treats her characters as realistically as she can. There's nothing clean cut about the flashback sequences, as instead of being separated by chapter, they are intercut to scenes of present day, creating an at first disorienting effect on the reader. Still though, the reader quickly settles into its rythms as it becomes apparent that the fictional characters remember past events in the same way as real people. The memories are disjointed and sporatic, meaningful and meaningless. This, coupled with Paul D's fear and Sethe's guilt and Denver's isolation and Beloved's parental abandonment, make for truly realistic characterizations. So when Morrison breaks the fourth wall during the final experience, the effect can especially jarring, reminding the readers that what they're reading isn't real, just the trials and tribulations of fictional characters. (though admitadly based upon real events, apparently).