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
Treehouse Of Horror VI; this one comes right after a Headless Horseman Krusty throws his own head at the screen, with the resulting blood splatter spelling out the title card
; The Grim Reaper sitting on the couch, and the family members dropping dead one after another (and on each other) as they rush to the couch*
Treehouse Of Horror X; said cavalcade consisting of Homer-in-the-box (THOH II), Alien-Maggie (THOH IX), Half-The Fly Bart and Witch-Marge (both from THOH VIII) where the Alien Maggie incinerates the Odd One Out, Lisa "as an axe victim", as soon as the latter questions what aliens have to do with Halloween
.
Screwed by the Network: Fox has been generally unable to schedule these episodes within the week before Halloween since buying the exclusive rights to the World Series in 2001.
This was even lampshaded in one special when the episode aired passed the Halloween holiday with Krang and Krodos stating it was near Christmas time.