The Imposter's revenge plan against Dan gets rather... intense. Please note the moment he disguised himself as Mr. Mumbles.. and them waited for Dan to wake up. All to drive him insane.
He was TERRIFYING in this episode. He knew Dan's mannerisms so well he even know when Chris would be standing where, and how Elise would react to a trap. And you can't say his plan wouldn't drive you nuts, either, having stashed phones all over Dan's apartment, and even a transmitter in his tooth to make him hear voices. DAMN.
The monster under the bed is pretty freaky until we find out it's all a clever ruse by Chris and Elise to get Dan to leave town.
The same episode suggests that Mr. Mumbles starved to death.
"Dan Vs. Technology", where Dan and Chris are trapped in the woods with a creepy guy who doesn't use technology, and end up trying to escape after he gets mad at them for destroying his technology free moving picture slide show of a fat woman in a bath tub. Seriously, he's got an axe, and after miles of driving away from him in a car, he's still chasing them.
Also from this episode, if Dan and Chris hadn't shown up, Elise could have remained as Ditmer's CyborgMind Control slave forever.
The boss when we find out she's actually a demon could easily be this...if it wasn't so hard to take her seriously. Dan's reaction to her pretty much cements it...until he goes kidnapping Chris. Sure, he wasn't really gonna sacrifice him, but we don't know this til afterwards...
Even before the reveal, the family of cannibals in Skiing are pretty damn creepy. They're just... Too nice.
The normally perky cruise instructor's Sanity Slippage in "The Family Cruise" is also unpleasant.