If multiple threads are running, the order that the threads are scheduled is nondeterministic. And if there are any timeouts anywhere in the code, the result is

If multiple threads are running, the order that the threads are scheduled is nondeterministic. And if there are any timeouts anywhere in the code, the result is nondeterministic due to the nondeterministic method that the kernel will schedule the process and environmental effects of whatever other programs are running at the same time or recently ran.