In Noita, the answer is a checkerboard.

In Noita (Nolla Games; Early Access in 2019, 1.0 in 2020) every pixel follows a tiny rule: if the spot below is empty, fall; if not, slide down to one side. Water does the same, then spreads sideways. Easy for one grain, hard for a whole world every frame, so the work is split between cores. But if two of them update the same pixel at once, in Petri Purho's words at GDC 2019, "all hell breaks loose and everything is destroyed."

Noita's fix: the world is cut into squares of 64 by 64 pixels, updated in four rounds, every other square across and down. A pixel may move only 32 pixels out of its square, so squares that run together never touch the same pixel. Purho calls it "a checker pattern of sorts." And each square keeps a dirty rect, so a square where nothing moves is skipped.

The tray, the grains and the race between four cores in this film are our own simplified version, built in code: our squares are 16 blocks wide with 8 out, the same proportions.

Which game's hidden rules should we cut open next?

Sources: Petri Purho, "Exploring the Tech and Design of Noita", GDC 2019. Petri Purho interviewed by Daria Loginova, "Noita: a Game Based on Falling Sand Simulation", 80.lv, 5 April 2019.
Sounds: Apple iMovie sound effects.