Determinism is an important part of computing.
2 + 2 = 4
Knowing that the computation of the above never changes gives the programmer the ability to reason about there machine in a scientific way. Two plus two is always equal to four!
Or is it?
With our molten iron core spinning within the earth, it creates a giant electromagnetic (EM) field that shields the planet. Very high up there, there is a cloud of stuff that is rejected and deflected from entering our atmosphere and ultimately us and our devices.
Mostly sunburn is the only problem coming from the heavens toward us, but there are other things to “look” for as well. Cosmic Rays are really charged particles believed to come from the stars exploding…
So what happens when one of these particles hits a semi-conductor like your RAM, CPU, SWITCH?
Bad stuff. It could flip a bit.
2 + 2 = 5
This could be a problem. Flip the right bit, you have a blue screen of death. Not a big deal to an entertainment device or non-mission critical systems. Very big deal for a self-driving car, or a power plant control and monitoring system.
Shielding can help, but shielding is expensive. I wonder how well those NeXT cubes with the magnesium cases do for shielding.