These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ensemble Darkhorse: Some of the monsters such as Count Burns, Devil Flanders, and King Homer. An unusual example is Krusty in vampire getup from Treehouse of Horror IX who, despite not really playing a role in the segment, has been seen in Treehouse of Horror merchandise and media.
Family Unfriendly Aesop: Ned teaches one to Bart, Lisa, Milhouse and Nelson about the seven deadly sins in Treehouse of Horror XVIII.
Treehouse of Horror III to Treehouse of Horror X featured several disturbingCouch Gag sequences, including: the Simpsons family appearing as skeletons*
Treehouse Of Horror III, also used as official promotional artwork for Treehouse Of Horror II
; erupting from the ground as zombies*
Treehouse Of Horror IV
; mismatched Frankenstein-esque monsters *
Treehouse Of Horror V; this one serves as a Self-Parody to another Couch Gag where normal Simpson family members have mismatched body parts, but while in that one they correct the mistakes, in this one they screw it up even more