If the player eats Buck, a shadow called The Follower will follow them for 7 minutes. They'll get sound cues such as an ominous noise when the screen shakes and music when The Follower is nearby. If the player is touched by The Follower, they'll die, but if they survive for the 7 minutes, The Follower will leave them alone. Also, Folly mentions in multiple conversations with other NPCs how she watches over their dreams, and she often torments them with their fears. This are some instances where someone in the game is stalked, so if I missed something, please add a comment!
If the player gives a kitty to the Unpleasant NPC a crunching noise plays, implying that Unpleasant ate the cat. Buck, who resembles a cat, can also be eaten by the player
Pilby's mother is trapped in another dimension, and it's unknown if she's alive or not. If Folly is in the elevator, she will psychologically torment Pilby and mock them that their mother is likely dead. It makes them cry.
In Jeremy's Domain, Jeremy will grab a player (or if there's only one player, an NPC), take them away and put them in a cage. The remaining player(s) have to save them but if the timer runs out and the kidnapped player/NPC isn't saved, they will die.
The Birthday Apartment floor has little plushies that can be destroyed by the player. If Poob (the owner of the apartment/plushies) is in the elevator, they will be visibly very upset for the rest of the game.
Many floors contain jumpscares, but there is a setting to turn them off. They will be replaced by a less scary simple png or removed entirely. Ex: Being caught by Reddy on 'Redball Diner', getting Fafa shows a regular orange, and Dolly from Mozelle's Strawberry Game is removed entirely.
On the Underground Subway floor, several NPCs will leave the elevator and deliberately stand on the tracks, letting themselves be hit and killed by the train. They can respawn, however, just like their other deaths.