In "War and Peaceville" Mina asks Trina if she and Corey are related and Trina responds with a "yuck". At first glance, this seems to be a somewhat standard response from Trina regarding her brother, but Word of God confirms Trina and Corey were adopted by a single father so they really aren't related.
In "Dueling Buttons", Corey became obsessed with defeating Trina in a video game after she won. According to Word of God, Corey has A.D.D. and O.C.D., so of course he would act this way after a loss!
The Bonkerton comet. When it comes around, literally anything can happen, and there's no controlling it.
At the end of "Hologroj", Trina snaps and and has an existential crisis, believing that nothing is real. This becomes pretty horrifying when we never actually see her recover - it's only assumed.
In "Grin Reaper" Torbo is revealed to be sucking happiness out of people, leaving them as piles of ashes. We see he wiped out three other towns (North Mountainfield, South Mountainfield, and Extra-South Mountainfield) before coming to Peaceville. In the end, we see a bunch of Peacevillians brought back to life by Grojband's song, but as far as we know, the citizens of the other towns affected are still dead.
In "Group Hug", Kate and Allie kidnap Corey by using 'Slumberberry' scented candles to knock him out. When he falls unconscious, he's in his pyjamas, but when he wakes in the groupies' bedroom, he's in his regular clothes. This doesn't seem like a big deal until you realize that Kate and Allie must have undressed and then dressed Corey again whilst he was asleep. That's more than a little creepy.