The Mr. Bucket episode marks the beginning of the horror episodes, especially with the Paranoia Fuel of the bucket stalking James as well as him drowning the bucket in a sink in a very disturbing manner. Mike even looks visibly discomforted.
Bootsy's and James's unexpected outbursts in the Shark Attack episode. Not only do they come out of nowhere, they sound genuinely angry during their respective lash-outs, as opposed to the humorous, joking rivalry dynamic they and Mike otherwise display. Worse yet, this video directly preceded the Dream Phone episode, indicating their uncharacteristic moments of anger were meant to foreshadow the upcoming major shift in tone.
The Dream Phone review is almost entirely made up of nightmare fuel. The phone turns out to be evil and hellbent on causing terror for James one late night, Bootsy is hanged in a closet, and Mike is bloodily stabbed. Then we realize that the phone was the murderer the whole time. Don't ask us how that works.
Not only that, but there's a twist ending where James seems to have gone insane, which brings up this question: Was the phone the murderer, or did James go crazy and kill Bootsy and Mike himself?
It's really Nightmare Fuel in a sense more so than Mr Bucket. While James in the past has toted Mr. Bucket to be more comedy horror, the Dream Phone episode has very little humor that could be interpreted as scariness and creepiness.
The Omega Virus episode shows a very different James than we've gotten used to, and he appears to be going insane from the isolation he's had for presumably a long time now, given that there appear to be multiple Jameses in the room. The creepiness is ultimately subverted at the end, which reveals all but one of the Jameses to be Terminators... or something.
Just his entrance in this episode can send chills down your spine, knowing what he may or may not have done in the previous episode... He climbs into the attic, looks around, and then toward the camera, remaining emotionless for a brief moment, before he puts on a VERY unnerving smile, and just casually says, "Let's play a game!" As he continues to talk, he clearly comes off as a psychopath... It's all so uncomfortable until finally the tension eases as he looks at the games...
In Lie Detector, when the fight promoter accuses James of murder, we get brief reshots of both the Mr. Bucket and Dream Phone episode. Each of them implying that James is nuts and that he hallucinated the Bucket and the Phone attacking him.
At the end of the Ouija / Domino Rally episode, the revived Mike and Bootsy assure James that nothing is wrong and nothing ever happened, repeating "nevermore" as the camera alternates between and zooms in on their faces. James simply adds "... quoth the raven, nevermore", sporting another very creepy smile and topping it off with a disturbing chuckle.
James' ritual, with all of the Black Speech and genuinely creepy atmosphere, especially with the part when he slits his wrist (although some of the scare factor can be alleviated with the fact that it's clearly ketchup coming out).
The Acid-Trip Dimension that James enters. Special note goes to the giant eye in the darkness as well as the returning appearances of Mr. Bucket and the Dream Phone.
The Full House/Urkel Game episode shows that James really didn't murder his friends after all! They never actually existed in the first place.
The Steve Urkel glasses/blindfold from the "Do the Urkel" game have an Uncanny Valley-esque creepiness about them. That creepiness becomes all the more apparent when Mike and Bootsie wear them as they reveal to James that they aren't real.
13 Dead End Drive. Holy fuck... if you thought the Mr. Bucket and the Dream Phone episodes were diving into the dark horror flick territory, this is where the show basically started using the submarine. Highlights include the vehemence of James during the deaths of his friends, and of course the whole Hangman murders subplot.
Perhaps the worst of it all was the graveyard scene, where James "plays" the Operation/Hangman games. It seemed like something out of a Torture Porn film, and the acting of Mike and Bootsy added up to the whole thing. Also, James just put things such as a bucket and a wrench inside Mike's Body while using a live battery wire to pull out them out just to make the game more realistic. James also cuts off Booty's limbs one by one with a chainsaw while commenting on the Fridge Logic of Hangman. Holy fuck indeed.