Comment by ratorx

I think the vast majority of SRE problems are in the “side effects” category. But higher level than the hardware-level side effects of the computer that you might be imagining.

The core problem is building a high enough fidelity model to simulate enough of the real world to make the simulation actually useful. As soon as you have some system feedback loops, the complexity of building a useful model skyrockets.

Even in “pure” functions, the supporting infrastructure can be hard to simulate and critical in affecting the outputs.

Even doing something simple like adding two numbers requires an unimaginable amount of hidden complexity under the hood. It is almost impossible for these things to not have second-order effects and emergent behaviour under enough scale.