Things that the players think are computers. Sorry for such a vague answer, but here's an example.

Suppose some people live in a Minecraft server, unaware there's anything else. Within the game, they build some computers (search for "working computer in Minecraft", it's been done many times).

They run their computers a long time and observe soft errors. The soft errors are caused by cosmic rays hitting the Minecraft server in the real world, but our heros don't know that. They attempt to protect their computers, moving them underground and setting guards to make sure no creepers get in, etc. But nothing they do stops the soft errors. Of course! What could they possibly do to stop soft errors caused by real cosmic rays hitting the Minecraft server?