9. Digimon Die Of Old Age - Digimon World
If you played Digimon World as a kid, there's a good chance you ended up getting pretty attached to your Digimon, enough that you were left thoroughly traumatised when they started dying of old age.
That's right - while you might've expected to be able to hold onto your Digimon forever in the game, they have a finite lifespan, and eventually perish no matter what you do.
You're forced to watch as their life force shatters and they're transformed into an orb of brilliant light, before being informed that the Digimon has "faded away."
It's a genuinely heartbreaking revelation, though it is somewhat cushioned by the fact that dead Digimon also give birth to a "Digitama" - an egg which allows them to be reborn, albeit with players having to raise them to adulthood all over again.
It's a lesson in the ephemeral nature of existence that nobody expected to learn while playing a Digimon game, of all things.