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.
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.