That's doesn't happen in a single thread, but e.g. asynchronous multithreaded code can spit values in arbitrary order, and depending on what you do you can end

That's doesn't happen in a single thread, but e.g. asynchronous multithreaded code can spit values in arbitrary order, and depending on what you do you can end up with a different result (floating point is just an example). Generally, you can't guarantee 100% reproducibility for uncooperative code because there's too much hardware state that can't be isolated even in a VM. Sure, 99% software doesn't depend on it or do cursed stuff like microarchitecture probing during building, and you won't care until you try to package some automated tests for a game physics engine or something like that. What can happen, inevitably happens. We don't need to be looking for such contrived examples actually, nixpkgs track the packages that fail to reproduce for much more trivial reasons. There aren't many of them, but they exist: https://github.com/NixOS/nixpkgs/issues?q=is%3Aopen+is%3Aissue+label%3A%226.topic%3A+reproducible+builds%22 https://github.com/NixOS/nixpkgs/issues?q=is%3Aopen+is%3Aiss...