You can't easily, automatically test concurrent code for correctness without testing all possible interleavings of instructions, and that state space is usually galactically huge.

It is very easy to write multithreaded code that is incorrect (buggy), but where the window of time for the incorrectness to manifest is only a few CPU instructions at a time, sprinkled occasionally throughout the flow of execution.

Such a bug is unlikely to be found by test cases in a short period of time, even if you have 1000 concurrent threads running. And yet it'll show up in production eventually if you keep running the code long enough. And of course, when it does show up, you won't be able to reproduce it.

That is, I think, what the parent commenter means by "luck".

This is similar to the problem you'll run into when testing code that explicitly uses randomness. If you have a program that calls rand(), and it works perfectly almost all the time but fails when rand() returns the specific number 12345678, and you don't know ahead of time to test that value, then your automated test suite is unlikely to ever catch the problem. And testing all possible return values of rand() is usually impractical.

What algorithm ? The whole idea is that algorithms are useless, and you should just write a bunch of tests and go with it

Yes, if I write stuff with locks, I shall ensure that my code acquires and releases locks correctly

This is completely off-topic with the original post;

Also, you cannot prove something by tests; Just because you found 100000 cases where your code works does not mean there is not a case where is does not (just as you cannot prove that unicorn does not exist) :)