The "Mirror" dream is also pretty unsettling, especially the variation where the Mii is just looking at themself when suddenly another Mii in the background of the mirror reflection slides up from the ground. The Mii looks behind them to see no one and starts shivering with fear, then look back around and the Mii has suddenly shown up in front of them waving, which scares the crap out of them. A scene that wouldn't at all be out of place in a slasher horror film.
Even further, the same type of room can actually be bought. Just an empty white void. In fact, it's one of the cheapest rooms in the entire game at only $20, which shows how much of a seller it is.
The Mii's reaction to eating their worst ever food: They suddenly turn grey and look terrified as a Scare Chord plays and they melt into the ground. It's the most scary when you feed a Mii their worst ever food for the first time. Subsequent times may become Funny Moments.
Some may be startled by the first note of the music that plays when a Mii is fed the food they find the worst for the first time. The Mii's reactions are also a smidgeon more realistic than when they are fed their "worst ever" food, so some others may deem the former scarier.
Deleting your save data makes your lookalike shrink down to nothingness while remembering their times with you. Doubles as a Tear Jerker.
A Mii's desperate cries of "I need to eat something! Anything!" can trigger people who are afraid of starvation, or just worried that they're neglecting that Mii.
When erasing a Mii, instead of the usual sad departure that a lot of other games do, they just say, "___ seems to have moved away somewhere..." That's kinda creepy in that it sounds like they're implying that they're lying and that something horrible has happened to that Mii.
The "Stretchy-Face Challenge" news. It involves a Mii's face becoming disturbingly wide, and it goes deep enough into Uncanny Valley that it becomes somewhat creepy.
If you visit a married Mii while they're in their house, they'll turn to look at you with a shocked expression. If you do this while they're playing peek-a-boo with a baby, their head will continue shaking back and forth as they do, making them look a bit like victims of demonic possession.
It is possible for a Traveler Mii you sent out to be erased, either because the owner of the island it's on chose to delete their game data or turn off StreetPass, (physical copies) the island's corresponding game copy got lost, stolen, or damaged, (digital copies) or the SD card and/or associated 3DS got lost. And unless the island owner is someone you know or somehow manages to get in contact with you, you will never know it happened.
After a breakup or if a Mii already in a relationship has their relationship stolen by another Mii who confesses their love and succeeds, what would usually be a Tear Jerker is made rather unnerving with this eerie song playing in the apartment of the one who initiates the breakup or the one who was cheated on.