There are several things that can make builds nondeterministic: * Many file formats, especially (but not exclusively) archives like .zip, embed timestamps in t

There are several things that can make builds nondeterministic: * Many file formats, especially (but not exclusively) archives like .zip, embed timestamps in them. * Absolute paths are also ridiculously easy to get embedded in the build artifact. * Other components of the user's environment can get embedded, including current locale, username, hostname. Some of these are more common than others. * Using an actual random number generator is more common than you might think (e.g., generating a cryptographic key during the build process), although still pretty rare. * Saying 'for file in directory' is usually sorted in inode order by default, which makes it inconsistent on different people's drives (although deterministic for a single user, generally). * Pointer addresses are nondeterministic. And it's easy to accidentally make a container that sorts by pointer address (e.g., std::map in C++). * In general, there's several features of programming languages that make it easy to accidentally inject nondeterminism in your program. Concurrency is obvious, but using standard containers like rust's HashMap or C++'s std::map combined with a "for every value in container" make it easy to get that nondeterminism. Compilers are not immune from being nondeterministic, and while it is a bug for them to be nondeterministic, it still happens.