The death screens. They come out of nowhere, they can't be skipped, and you're completely powerless to do anything. You have to sit and watch your pet die.
The Game Boyadaptation might be the most shocking and sudden. It starts with the screen flashing rapidly, the Tamagotchi looks sick, a skull floats over its head, and in the background, a slowing heart-rate monitor can be heard. It slows down gradually until it flat lines.
What's worse is that this can happen at any time and for any given reason, which means the death screen may pop up when you don't expect it and startle you with its flashing lights, high-pitched sounds and the dead Tamagotchi (especially if you, for example, played a game with the Tamagotchi a moment before).
The third game (Tamagotchi Osucchi to Messuchi) had a different death screen than in the previous two games, which featured a deceased Tamagotchi that is about to be buried next to a headstone with a large cross while the spirit in a different pose flashing in the dark sky and the music was darker and creepier compared to the previous two games.
On the Japanese toys, from the Keitai onward, the pet is visited by the Tamagotchi version of The Grim Reaper. And you have to watch as the pet cries until it's escorted away.
The Anime and Related Adaptations
Terrifying! Mametchi the Vampire. Mametchi wants to find a sister to be with him. However, when Lovelitchi runs away holding Memetchi's hand, Mametchi chooses Makiko and even bites her.
Later on, to attempt to try to get all of the Nightmare Fuel out, every sister (even Kuchipatchi, although he's a male) throws pieces of garlic, which has Mametchi screaming.
After Mametchi comments about the garlic, the Nightmare Fuel comes back as Mametchi looks like he is vomiting purple.
All the events in the first movie thanks to Blackholetchi, a black hole Tamagotchi - coupled with the fact that Mametchi was scared of the dark and almost got himself sucked into the black hole. Becomes less scary when Blackholetchi does wake up; he resembles a giant space squid and has an equally goofy voice.
That awfully eerie robot clone of Papapianitchi that Mametchi created in the 25th episode of Tamagotchi! Yume Kira Dream. Especially its crudely made facial features. Mametchi's friends are rightfully scared.