Another fun one for reproducibility -- rounding. It can happen when compiled for multiple architectures, compiling a second time with different optimizations, .

Another fun one for reproducibility -- rounding. It can happen when compiled for multiple architectures, compiling a second time with different optimizations, .... Things like normal random numbers are generated through _some_ kind of rejection sampling, and crossing that threshold from different rounding behavior will give you a wildly different result. Even if all your application code is tolerant of small perturbations, the result isn't reasonably reproducible. Another way things go wrong with the prng tree idea (assuming the implementation is correct and you actually have independent streams) is race conditions in other parts of your code. E.g., say you have two workers reading a queue and using fancy tree-based randomness. You don't have "race" conditions (data races) in safe Rust without a compiler bug, but the language doesn't define which worker will read which item from the queue. You wind up using a different part of the stream for different inputs.